Plant Kingdom

(A) is wrong: $Equisetum$ is a homosporous pteridophyte; its spores germinate into a free-living, independent gametophyte (prothallus). The female gametophyte is retained on the sporophyte only in heterosporous forms such as $Selaginella$ and in seed plants.
(B) is correct: $Ginkgo$ is a gymnosperm; in gymnosperms the male gametophyte is highly reduced, is confined to the pollen grain and has no independent free-living existence.
(C) is wrong: the sporophyte of $Riccia$ (a liverwort) is the simplest, consisting only of a capsule, whereas the sporophyte of $Polytrichum$ (a moss) is more elaborate with foot, seta and capsule.
(D) is wrong: sexual reproduction in $Volvox$ is oogamous (a large non-motile egg and a small motile male gamete).
(E) is wrong: the spores of slime moulds possess true walls and are extremely resistant.
Only statement (B) is correct, so the number of correct statements is one.

Mustard is an angiosperm; its embryo sac has an egg apparatus made of one egg cell and two synergids, so the pair is correct.
$Ginkgo$ is a gymnosperm; the female gametophyte of gymnosperms bears archegonia, so the pair is correct.
Viroids are infectious agents made of free, low molecular weight RNA without a protein coat, so the pair is correct.
$Salvinia$ is a heterosporous pteridophyte: its microspores and megaspores give rise to highly reduced male and female gametophytes that develop within the spores, so it does not form a free-living, independent prothallus as homosporous ferns do.
Hence the wrongly matched pair is $Salvinia$ – prothallus.