<u>A gerund </u>is a verb form which acts as a noun in a sentence. An example is provided in option A), where "climbing" is used as a noun.
<u>An infinitive</u> is a verb form which consists of the word "to" and the base verb form. For instance, "to complete" in option B) is an infinitive.
Finally, <u>a participle</u> is a verb form used as an adjective or a verb. "Roasting" (option C) is a participle which is used as an adjective, since it describes the noun "garlic."