Final answer:
The gerund in the sentence 'Drooling is rude' is 'drooling,' and it functions as the subject of the sentence since it is the topic being discussed.
Step-by-step explanation:
In the sentence 'Drooling is rude,' the gerund is 'drooling.' A gerund is a verb form that ends in -ing and functions as a noun in the sentence. In this specific sentence, 'drooling' acts as the subject of the sentence. This is because it takes the place of a noun as the entity performing the action or the topic being discussed. The verb in this sentence is 'is,' which acts as a linking verb connecting the subject to the subject complement 'rude' which describes the subject. Therefore, the correct answer to the question is that the gerund 'drooling' acts as the subject.