Which one of the following has an open circulatory system ?

In an open circulatory system the blood pumped by the heart passes through large vessels into open spaces or body cavities called sinuses, so the tissues are bathed directly in the blood.
It is found in arthropods and in most molluscs.
Periplaneta (the cockroach) is an arthropod; its body cavity is a haemocoel filled with haemolymph, and its blood vessels are poorly developed. So it has an open circulatory system.
Pheretima (the earthworm) is an annelid with a closed circulatory system.
Hirudinaria (the leech) is also an annelid; its blood flows in closed haemocoelomic channels.
Octopus is a cephalopod mollusc; unlike other molluscs, cephalopods have a closed circulatory system.
So the animal with an open circulatory system is Periplaneta.