**Core Concept:** World Health Day is an annual event organized by the World Health Organization (WHO) to raise awareness on a specific health-related theme.

**Why the Correct Answer is Right:** The correct answer is **Option D:** Universal Health Coverage (UHC). UHC is a central concept in the WHO's agenda, aiming to ensure that all individuals and communities receive the health services they need without suffering financial hardship when paying for those services. UHC addresses the need for equitable health care access and protection from financial harm, which is essential for overall health improvement and development.

**Why Each Wrong Option is Incorrect:**

A. **Option A (Noncommunicable Diseases):** While noncommunicable diseases (NCDs) are a pressing concern globally, they are not the theme for World Health Day 2013. Instead, the day focused on UHC.

B. **Option B (Mental Health):** While mental health is also crucial, the specific theme for World Health Day 2013 was UHC, not mental health.

C. **Option C (Access to Medicines):** While access to medicines is important, it is not the central theme of World Health Day 2013. Instead, the focus was on UHC, which encompasses access to medicines and other health services.

**Clinical Pearl / High-Yield Fact:** The choice of a specific theme each year helps raise awareness on important health issues and promotes global coordination for improved health outcomes. The WHO selects a theme that directly correlates with its mission and goals. In the case of UHC, it emphasizes the need for equitable health care access and protection from financial harm, ultimately benefiting the overall health of individuals and communities.