Answer:
The best option is letter A) rowing is a gerund functioning as a noun.
Step-by-step explanation:
Gerunds are formed by adding -ing to the base form of a verb. Even though they look like verbs, gerunds function as nouns in a sentence, which means they can be the subject, the object etc. In the sentence "Rowing in the lake is one of my father's favorite Sunday afternoon activities," the part "rowing in the lake" is a gerund phrase. Gerund phrases have a gerund and any modifiers or objects related to it. In this case, the gerund phrase functions as the subject of the whole sentence, being connected to its predicate by the liking verb "is". Therefore, it is safe to choose letter A as the correct option: rowing is a gerund functioning as a noun.