The correct answer is D. Most animals thrive where they can find food easily.
Adverbial clauses act as adverbs - they answer the questions of where, when, why, etc. and always have adverbial functions. This is why A and C are not adverbial clauses - because both clauses have noun functions - both of them are used as objects rather than adverbs. B is incorrect because it is a noun clause, not an adverbial one.
Therefore, D is correct. Here, the adverbial clause is where they can find food easily.
