Which words in the sentence are the complete infinitive phrase?

It can be very hard to speak in public.

a. very hard

b. to speak in public

c. it can be

d. to speak

An infinitive phrase is often indicated with 'to' followed by the simple form of a verb. So in this sentence, the infinitive phrase will be (D) to speak.

This is because 'to' is followed by the simple form of a verb 'speak'.