Question

Which words in the sentence are the infinitive?

He believes his grandfather will decide to visit.

A.
He believes

B.
his grandfather

C.
to visit

D.
will decide

Answer 1

An Infinitive phrase is a phrase that starts with the word to so it would be C.
Hope I helped : ) 

Answer 2

The infinitive words in the sentence "He believes his grandfather will decide to visit." are C. to visit