I would say this is a "noun phrase" because you could say... In the experiment the stimulants caused the plants to grow.
Answer: C) adjectival phrase.
Explanation: an adjective is a word that describes or modify a noun or a pronoun in a sentence. In the given sentence we can see an example of an adjectival phrase, because the words "in the experiment" are modifying the noun "stimulants." It isn't a noun phrase because it doesn't function as a subject, object, or prepositional object. It isn't a verb phrase because it doesn't indicate an action, and it isn't an adverbial clause because it doesn't modify a verb or an adverb.
