Gerund phrases always end in –ing and are used as nouns. Option A is correct. Gerunds or gerund phrases ending in -ing are forms of a verb that when used in a sentence act as nouns. In other wordst, it s a noun ending in -ing to have the sense of a noun intead of a verb. For example in the sentence 'Running is healthy' , the gerund in this particular example is 'running' since it plays the role of the subject.
