This is correlation/causation question. I would not necessarily recommend diagramming it since there is no conditional logic. The stimulus tells us that there is a correlation between education and good health, and concludes that education and informed lifestyle choices must be the cause of good health. The issue with this argument is that correlation could be indicative of but never conclusive of causation, thus it is equally likely that a third confounding variable is responsible for both - high education level and good health, for example one's income. (D) accurately expresses this flaw as the argument ignores the possibility of a third variables and concludes that high education is the cause of good health.