A person who exercises vigorously every day has less body fat than an average person to draw upon in the event of a wasting
Why this is right
The conclusion is not an explicit comparison, but it is making a recommendation. Recommendations are implicitly comparisons. For example, if I say, "If you're going to Martha's, you should take the 101, not the 134", I'm saying taking the 101 is better than taking the 134. The conclusion of the original argument could be heard as "you should drive a small car". So this is saying "exercising vigorously every day is better than not doing so". The negative? You'll burn off all your body fat, and in doing so you'd be more vulnerable were you to get a wasting illness (a disease that steadily eats away at your stores of fat). A 200 lb. man could contract a wasting illness and ultimately survive it, even though the illness may have decimated him down to 150 lbs. Meanwhile a frail 150 lb. man who contracts a wasting illness might not survive the illness. The positive? You're way less likely to contract a wasting illness. This beats the other answers because it matches the original argument, in terms of the relationship between the positive and the negative. The positive of vigorous daily exercise (way less likely to contract a wasting illness) makes the negative (you'd be screwed if you got a wasting illness) less likely to matter.
Skill tested: Parallel · how this choice captures the argument's function is the move to repeat next time.