The argument is drawing a causal conclusion from correlation. I.e. that eating foods rich in triglycerides causes increased risk of heart disease, because of the correlation between high levels of triglycerides in the blood with heart attacks.
But just because we have correlated high levels of triglycerides in blood with heart attacks does not mean that intake of triglycerides is the cause or even a cause at all. For all we know, it could be that heart disease causes high levels of triglycerides in the blood, or a third factor such as stress causes both.
Answer choice D weakens the argument because it brings forth the possibility that the relationship is reversed.