Which sentence contains an infinitive or infinitive phrase?

a. Find me tomorrow and tell me all about it.

b. Alice is going to listen to the sound of the crickets.

c. A group of children walked to the library.

d. Chirping birds entertained us as we sat

A good way to know whether there is an infinitive is to see whether there is the word "to" in the sentence.

So b. and c. are good candidates.

in C. however to is not part of the verbal phrase:

so the correct answer is b: and the infinitive is to listen