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".