**Core Concept**
The Hopewood House study is a landmark investigation that examined the effects of diet on the prevention of coronary heart disease (CHD). It was a prospective cohort study that aimed to determine the relationship between dietary patterns and the risk of CHD.

**Why the Correct Answer is Right**
The Hopewood House study, conducted in Australia, focused on the dietary habits of children and their impact on the development of CHD in adulthood. The study found that a diet rich in saturated fats and low in fiber was associated with an increased risk of CHD. The researchers observed that children who consumed a diet high in meat and low in plant-based foods were more likely to develop CHD later in life. The study's findings highlighted the importance of a balanced diet in preventing CHD.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the Hopewood House study focused on the dietary habits of children and their long-term effects on CHD risk, not on the treatment of existing CHD.
**Option B:** This option is incorrect because the study was a prospective cohort study, not a randomized controlled trial (RCT).
**Option C:** This option is incorrect because the study did not specifically examine the effects of sugar on CHD risk.

**Clinical Pearl / High-Yield Fact**
The Hopewood House study demonstrates the importance of early dietary interventions in preventing CHD. A diet rich in fruits, vegetables, and whole grains, and low in saturated fats, can help reduce the risk of CHD in adulthood.

**Correct Answer:** D. Dietary habits of children in the Hopewood House study were examined to determine their impact on the development of coronary heart disease.