Final answer:
To clarify the sentence and reduce ambiguity, 'skydiving' should replace the pronoun to indicate that it is more fun compared to vacuuming, despite both activities burning equal calories.
Step-by-step explanation:
To reduce ambiguity in the sentence, the best choice is option 2, skydiving. The revised sentence would read: "Although vacuuming and skydiving burn an equal number of calories, skydiving is arguably more fun." This alteration clarifies that it is skydiving, not vacuuming or burning calories, that is considered more enjoyable.