The correct answer is A: It is a gerund functioning as a noun.
Gerunds are words formed from verbs. They function as nouns and end in -ing. Gerunds are used in sentences that convey the meaning of real or complete actions.
On the contrary, infinitives are verb forms that do not end in -ing. They are formed with -to preceding the verb form. They are used in sentences that convey the meaning of unreal or future actions.