Which sentence contains an adjective clause?
a. others, such as flies, have only two wings.
b. all insects have six legs, two antennae, and three body parts.
c. most insects have compound eyes that are actually thousands of tiny eyes.
d. some insects, such as grasshoppers, have four wings.

A clause must have a verb. An adjective clause is a dependent clause, which cannot exist on its own, it has to be a part of the main clause. So, having this in mind, the only adjective clause in these examples is 'that are actually thousands of tiny eyes', therefore, C is the correct answer.