The diet described involved a majority of calories from protein rather than from carbs. Whether a different kind of diet helps people “feel fuller” has no clear impact on whether the ad’s diet will help people lose weight.

We know the group who “lost the most weight” got more calories from protein than from carbs. That doesn’t exclude the possibility that some people with a different diet also lost a lot of weight. They just didn’t lose the most weight.

(C) suggests that exercise is another factor in weight loss. The author never assumed otherwise. The ad is merely arguing that following the ad’s diet will cause weight loss. Whether exercise also leads to weight loss doesn’t affect whether the diet leads to weight loss.

If this possibility is true, that means not everyone on the high-protein + big meal early diet lost weight. This shows why we can’t conclude that “anyone” on the ad’s diet will “be sure to” lose weight.

(E) describes people on a different kind of diet that isn’t what the ad describes. What’s true about those people has no affect on what will happen to people following the ad’s diet.