**Core Concept**
The question requires knowledge of the organizational structure of the World Health Organization (WHO), a specialized agency of the United Nations responsible for international public health.

**Why the Correct Answer is Right**
The Director-General of the WHO is the chief of the organization and is responsible for leading its efforts to promote health, keep the world safe, and serve the vulnerable. The Director-General is elected by the World Health Assembly (WHA), the supreme decision-making body of the WHO, for a five-year term. The Director-General is assisted by a team of senior officials and advisors in implementing the WHO's programs and policies.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because the Director-General is not elected by the World Health Assembly's Executive Board, but rather by the WHA itself.
**Option B:** Incorrect because the Regional Directors are responsible for leading the WHO's regional offices, but are not the chief of the organization.
**Option C:** Incorrect because the WHO's Secretariat is responsible for implementing the organization's programs and policies, but is headed by the Director-General, not a separate individual.

**Clinical Pearl / High-Yield Fact**
The WHO is headquartered in Geneva, Switzerland, and has six regional offices located in Africa, the Americas, South-East Asia, Europe, Eastern Mediterranean, and the Western Pacific.

**Correct Answer: D. The Director-General of the WHO.**