Most animals thrive where they can find food easily
Answer: D
Explanation
It is the correct answer because an adverbial clause or adverb clause describes group of words meant to function as a verb in a sentence.
An adverb qualifies a verb or other words excluding nouns.
It also add details to the sentence and improves the meaning.
The adverb clause should contain a verb and the subject.
In the question the subject is find food and the verb is thrive while easily as an adverb adds more meaning to the sentence.