Identify the sentence that contains an infinitive.
a. some parents think the new advertising campaign is sending the wrong message to young girls.
b. some reality tv stars have made the transition to movie careers.
c. would you be interested in a discount in exchange for filling out this survey?
d. the new mayor promised to look into the university's parking problem.
im stumped : 3

The correct answer is option D. The infinitive "To look"

An Infinitive verb is a verb usually preceded by the word "to" or it may also be used only with the base form of the verb. It is a verb which has not been conjugated and therefore is not linked to a particular subject or pronoun.

Note that in options A, B and C, all the verbs used are conjugated:

Parents think ( present simple from the infinitive to think), Campaign is sending (present participle from the infinitive to send); Stars have made (Past perfect from the infinitive to make). In option C we also can notice the use of the gerund after a preposition:  "for filling out".

what he !