This weakens the author's argument. It tells us that the reason the volunteers on the low-protein, high-carb diet appeared to lose less weight was not necessarily because they didn't lose as much fat, but because they were retaining more water. This potentially invalidates the comparison that the author uses to reach her conclusion.

The conclusion recommends both eating much protein and shunning carbohydrates. Just knowing that some people eat much protein and still gain body fat doesn't weaken this claim. It's possible these people gain weight because they eat lots of carbohydrates as well.

The conclusion is about losing fat, not losing weight generally. This answer choice doesn't weaken the argument, it just explains another way for a high-protein, low-carb diet to help people lose fat.

If anything, this strengthens the author’s argument. The high-carbohydrate volunteers were exercising and yet still didn’t lose as much weight as the high-protein volunteers.

This doesn't weaken the argument. We're interested in the effects of following these specific diets, not in what happens once people go back to their normal diets.