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.
it is basically the same thing just one is more proper than the other. Kind of like how people in the south speak one way and people from the north speak totally different.
