This question makes no sense. As saury2k points out, the question doesn't even make clear if the 100 people were all eating broccoli, and their heart disease rate was compared with the general population, or if only some of the 100 people were eating broccoli, and those people were compared with the rest of the 100 who did not eat broccoli. If we don't even know how the study was conducted, how can we know how to weaken its conclusion?
Answer C is only potentially right if everyone was eating broccoli. Then the fact that those people also jogged might make them different from average people in another respect besides broccoli-eating, and the jogging might account for their lower incidence of heart disease. But you'd need to know two things the question doesn't tell us: does jogging reduce heart disease, and is 30 minutes of jogging an above or below average amount of jogging? Without that information, there's no way to tell if C weakens the argument. And if C is right because all 100 people were eating broccoli, then B is right too. If these people are all from the same town, there are dozens of reasons (cleaner drinking water, genetic commonalities, etc, etc) that these people might be at lower risk of heart disease than are average people. It's a biased sample, and you can't generalize from a biased sample.
There's no value in studying questions like this.