Answer: B) Sailing is my aunt and uncle’s favorite activity.
Explanation: A gerund is a noun formed from a verb which refers to an action, process, or state. From the given options, the sentence that contains a gerund (a verb acting as a noun) is the corresponding to option B: Sailing is my aunt and uncle's favorit activity, because the verb "sailing" is functioning as a noun, specifically as the subject of the sentence (the one that performs the action of the verb).