Locomotion and Movement

Fibrous joints – e.g. flat skull bones fused by sutures; do not allow any movement Cartilaginous joints – e.g. between adjacent vertebrae; permit limited movement Hinge joints – e.g. knee; help in locomotion Ball and socket joints – e.g. between humerus and pectoral girdle; allow rotational movement Correct match: A-III, B-I, C-IV, D-II.

Synovial joints have a fluid-filled synovial cavity between the two bones and allow considerable movement. The pivot joint between atlas and axis is an example.

Gliding joints are synovial joints in which the flat articular surfaces slide over one another.
They occur between the zygapophyses (articular processes) of successive vertebrae, between the carpals and between the tarsals. So this pair is correctly matched.
Fibrous (immovable) joints occur between the flat bones of the skull (sutures), not between the phalanges.
The joints between the phalanges are hinge joints, and the joints between the vertebral bodies (with intervertebral discs) are cartilaginous joints.
So the skull bones and the vertebrae are wrongly matched in the other options.
So the correctly matched pair is gliding joint - between zygapophyses of the successive vertebrae.

Cartilage and the cornea are both avascular, i.e., they have no blood vessels. Being living tissues, they still need oxygen, which they obtain by diffusion (cartilage from the surrounding perichondrium, the cornea from the air and the aqueous humour). This pair is correctly described.
The knee joint is formed between the femur and the tibia (with the patella in front); the fibula does not take part in it.
The shoulder joint is a ball and socket joint, but the elbow joint is a hinge joint.
An adult has 8 premolars and 12 molars, i.e., 20 in all, but they are not all 3-rooted (premolars are mostly single- or double-rooted, lower molars are 2-rooted).
Hence the correctly matched pair is cartilage and cornea.

Fibrous joints do not allow any movement; the flat skull bones fuse end-to-end with the help of dense fibrous connective tissue in the form of sutures to form the cranium. The parietal and temporal bones are cranial bones, so they are joined by a fibrous joint. This description is correct.
The first vertebra is the atlas, which articulates with the occipital condyles; the axis is the second vertebra.
The floating ribs are the 11th and 12th pairs; the 8th, 9th and 10th pairs are vertebrochondral (false) ribs.
The glenoid cavity is in the scapula and articulates with the head of the humerus; the thigh bone (femur) articulates with the acetabulum of the pelvic girdle.
Hence the correct description is that the parietal and temporal bones are joined by a fibrous joint.

Muscle contraction is initiated by a signal sent by the CNS via a motor neuron (not a sensory neuron, so A is incorrect). The neurotransmitter (acetylcholine) released at the neuromuscular junction generates an action potential in the sarcolemma (B is correct). Increased $Ca^{2+}$ binds with troponin on actin filaments, which unmasks (not masks) the active sites for myosin (so D is incorrect as stated), and the myosin head uses energy from ATP hydrolysis to form cross bridges (E is correct). So the correct statements are B, C and E only.

Joint between humerus and pectoral girdle is a Ball and Socket joint (A-II); Knee joint is a Hinge joint (B-III); Joint between atlas and axis is a Pivot joint (C-IV); Joint between carpals is a Gliding joint (D-I).

Gout is caused by accumulation of uric acid crystals (a-iii); Osteoporosis is caused by decreased levels of estrogen (b-i); Tetany is caused by low $Ca^{2+}$ ions in the blood (c-ii); Muscular dystrophy is a genetic disorder (d-v).

Palm bones are Metacarpals (A-II); Wrist bones are Carpals (B-III); Ankle bones are Tarsals (C-IV); Digit bones are Phalanges (D-I).

Each half of the pelvic girdle is a coxal bone, formed by the fusion of three bones: ilium, ischium and pubis. This matching is correct.
The pyrimidines are cytosine, uracil and thymine; thiamine is vitamin $B_1$, not a nitrogenous base.
The ear ossicles are malleus, incus and stapes; the cochlea is the coiled part of the internal ear.
Actin and myosin are muscle proteins, but rhodopsin is the visual pigment of the rod cells of the retina.
Hence the correct matching is ilium, ischium, pubis – coxal bones of the pelvic girdle.

In the resting state the thin actin filaments from both sides do not meet at the centre of the A-band. This central part, with only thick filaments, is the H-zone.

Troponin is a complex protein present at regular intervals on the thin (actin) filament, and myosin is the polymerised protein forming the thick filament; both are proteins of striated muscle. This pair is correctly categorised.
Secretin is a peptide hormone of the gut, but rhodopsin is the visual pigment of the rods of the retina, not a hormone.
Calcitonin is secreted by the thyroid, but thymosin is secreted by the thymus.
Pepsin is a digestive enzyme of the stomach, but prolactin is a hormone of the anterior pituitary.
Hence the correctly categorised pair is troponin and myosin – complex proteins in striated muscles.

According to the sliding filament theory, during contraction the thin (actin) filaments slide over the thick (myosin) filaments towards the centre of the sarcomere. The filaments themselves do not shorten.
The A-band equals the length of the thick filaments, so it remains constant.
The H-zone (part of the A-band without overlap) decreases, and may disappear.
The I-band decreases, and the Z-lines come closer, so the sarcomere shortens.
So both 'the length of the H-zone decreases' and 'the length of the A-band remains constant' are correct.
Note: the answer key of the source accepts both of these statements, so both are marked correct here.

The pterygoid is a bone of the skull (in frogs and other vertebrates it forms part of the palate; in humans the pterygoid plates are part of the sphenoid bone).
The coracoid is part of the pectoral girdle, the arytenoid is a cartilage of the larynx, and the atlas is the first cervical vertebra.
So the bone related to the skull is the pterygoid.