Answer:
a. Noun
Explanation:
An infinitive phrase is a verbal expression of an action that combines to + a verb. The infinitive phrase could function as a noun, adjective or adverb in a sentence. When functioning as a noun in a sentence, the infinitive phrase provides details on what action was performed.
In the sentence above, the question, what is your responsibility?, is answered by the infinitive phrase, "to make good grades in school". Therefore, the infinitive phrase in this sentence functions as a noun.
