Which BEST identifies and explains the function of the word gardening as it is used in this sentence?

Gardening is a pleasant and productive pastime for people of all ages.
A) In the selected sentence, the word gardening is a gerund, and the gerund phrase functions as the subject.
B) In the selected sentence, the word gardening is a gerund, and the gerund phrase functions as a direct object.
C) In the selected sentence, the word gardening is a gerund, and the gerund phrase functions as a subject complement.
D) In the selected sentence, the word gardening is a gerund, and the gerund phrase functions as the object of the preposition.