Question

Which sentence has an infinitive that functions as an adverb?

Answer 1

You did not include your choices.

The (present) infinitive is the "to" form of the verb. Things which modify a verb act as adverbs. So you are looking for a "to" form verb which tells how, why, when, etc. the action of another verb occurs.

Julus went to the field to practice soccer.

"To practice" acts as an adverb because it tells why he "went." But "to the field" is not an infinitive because it is not the "to" form of a verb. (It is a prepositional phrase.)