The following sentence contains an infinitive phrase:
C. He visited Congress to gather support for his proposal.
An infinitive phrase is a group of words that uses an infinitive (“to” + verb). An infinitive is a verbal (a word that expresses action); so, an infinitive phrase has the same role of expressing action in a sentence. An infinitive phrase is just a part of a sentence, working like a noun, adjective, or adverb.
In this case, is “to + gather”.