Answer:
D). Astronomers discovered a quasar 12 billion light years away in 1991.
Step-by-step explanation:
A Prepositional phrase comprises of a preposition followed by an object(that could be a pronoun or noun, etc. and it's modifier. This phrase usually functions either as an adjective or as an adverb. As per question, the sentence among the given options that carries a prepositional phrase functioning as an adverb phrase is displayed through option D as it contains the prepositional phrase('away in 1991') that modifies the verb 'discovered' and also answers the question 'when?'. Therefore, option D is the correct answer.