Which words in the sentence make up the infinitive phrase?

Before the show, they went to find the water fountain.

a. to find

b. to find the water fountain

c. before the show

d. went to

The words in the sentence "Before the show, they went to find the water fountain" that make up the infinitive phrase are:

b. to find the water fountain

The infinitive phrase is a grammatical construct which includes the infinitive form of the verb plus any modifiers and complements.

In this sentence, the infinitive form of the verb is "to find"; the complement is the object and, in this case, is "the water fountain".

There is no modifier in the above sentence. The modifier is usually an adverb