Answer: infinitive
Step-by-step explanation:
The underlined phrase is an infinitive. An infinitive is referred to as a verb but which when used doesn't act as a verb rather the verb performs the function of either a noun, adjective or an adverb.
Infinitive is formed by adding "to" to the verb e.g. to read, to sing, to travel, to play etc
Therefore, the correct option is C.