This suggests physical activity is also a causal factor in heart disease. But this doesn’t suggest high fat consumption isn’t also a causal factor. (A) could have been correct if we knew that people with high blood triglyceride on average exercise a less than others.

Other diseases aren’t relevant to whether triglyceride levels are a causal factor in heart disease.

This strengthens the connection between consumption of alcohol/sugar and triglyceride. This is consistent with the author’s reasoning.

This suggests an alternate explanation for the correlation between higher blood triglyceride levels and heart attacks. If heart disease interferes with triglyceride metabolization, it would lead to higher blood triglyceride. This suggests the cause and effect could be reversed.

This has no impact, because we don’t know whether the people on these low-fat and low-sugar diets are more or less likely than others to have heart attacks.