Assertion: Acraniata is a group of organisms that do not have distinct cranium. Reason: It includes small marine forms without a head.

(c) The assertion is correct but the reason is incorrect.
Acraniata refers to organisms that lack a distinct cranium (skull). This includes species like lancelets (amphioxus), which are small marine invertebrates. While they lack a cranium, they do not necessarily have to be “without a head” as suggested by the reason. Many of these organisms have distinct body segments but no cranial structures.