The correct answer is A. making the class laugh constantly
Step-by-step explanation:
A gerund is a word that ends in -ing and derives from a verb form, for example, crying, walking or studying. Moreover, a gerund phrase refers to a set of words that begin with a gerund and together act as a noun because this is the main function of gerunds and gerund phrases. This implies that in the sentence presented the gerund phrase is "making the class laugh constantly" because this phrase contains the gerund "making" and also these words act as a noun by describing an activity performed by Billy.