**Core Concept**
The World Health Organization (WHO) advocates for dietary changes to prevent heart diseases, focusing on reducing the intake of harmful fats and cholesterol while maintaining essential nutrients. This approach aims to lower the risk of cardiovascular disease, a leading cause of mortality worldwide.

**Why the Correct Answer is Right**
The WHO recommends reducing dietary fat intake to 20-30% of total caloric intake (Option A) to minimize the risk of heart disease. This guideline emphasizes the importance of limiting fat consumption to prevent excessive calorie intake and reduce the risk of high cholesterol and triglyceride levels. Additionally, the WHO suggests limiting saturated fat intake to less than 10% of total energy intake (Option C) and reducing dietary cholesterol to below 100mg per 1000kcal per day (Option D). However, there is no evidence to support a decrease in complex carbohydrate consumption as a preventive measure for heart disease.

**Why Each Wrong Option is Incorrect**
**Option A:** Reducing fat intake to 20-30% of caloric intake is a recommended dietary change by the WHO to prevent heart diseases. This option is not the correct answer because it is actually a recommended guideline.
**Option C:** Limiting saturated fat intake to less than 10% of total energy intake is a crucial dietary change advocated by the WHO to prevent heart diseases. This option is not the correct answer because it is actually a recommended guideline.
**Option D:** Reducing dietary cholesterol to below 100mg per 1000kcal per day is a recommended dietary change by the WHO to prevent heart diseases. This option is not the correct answer because it is actually a recommended guideline.

**Clinical Pearl / High-Yield Fact**
The WHO's dietary guidelines for heart disease prevention emphasize a balanced diet with reduced fat and cholesterol intake, while maintaining adequate complex carbohydrate consumption. This approach helps to reduce the risk of cardiovascular disease and supports overall health and well-being.

**✓ Correct Answer: B. A decrease in complex carbohydrate consumption.**