The answer is C. mollusk.
Among the all choices, echinoderm and annelid are segmented animals. So, it leaves flatworm and mollusk as possible answer. However, flatworms have no a free-swimming larval stage. Mollusk is an unsegmented animal with a free-swimming larval stage we were looking for. After eggs production, from them trochophore larvae can emerge. This trochophore larvae is planctonic which means it is a free-swimming.
