Final answer:
The gerund phrase in the sentence is 'water skiing', which functions as the subject. Sailing and swimming are also part of the compound subject but are not gerunds in this context. The correct answer is option a) Water skiing; Subject.
Step-by-step explanation:
The gerund phrase in the sentence "Water skiing, sailing, and swimming are my favorite summer sports." is water skiing. This gerund phrase is functioning as the subject of the sentence. We know this because it is what is being talked about; water skiing is being identified as part of what constitutes the writer's favorite summer sports. In this case, sailing and swimming are also part of the compound subject, but they are not functioning as gerunds within the context of this question. Hence, the correct answer is option a) Water skiing; Subject. It should be noted that the phrase "my favorite summer sports" functions as a subject complement, giving us more information about the subject, but is not a gerund phrase.