Which sentence uses an infinitive as an adverb? a) my favorite board game to play is stored in the basement. b) nicole wants to learn the art of rhetoric before next year. c) dave's favorite thing to do is to walk his dog every evening. d) we met after school to study for the big math test next week.

In the above sentences, a sentence which uses an infinitive as an adverb is D. We met after school to study for the big math test next week.

Adverb Infinitive is used to modify the verb. As in the above sentence infinitive 'to study' modifies the verb 'met'.

Also, the infinitive adverb answers the question of 'why?'. In the above phrase the question " why did they meet after school?" - " to study for the big math test."

This way, Adverb Infinitives can be identified.