**Core Concept**
The World Health Organisation (WHO) Day is a significant event celebrated globally to promote health awareness and global health efforts. This day commemorates the founding of WHO, which plays a crucial role in shaping global health policies and initiatives.

**Why the Correct Answer is Right**
The correct answer is 7th April, as it marks the anniversary of the establishment of WHO in 1948. This date is significant, as it represents the foundation of WHO's mission to promote health, keep the world safe, and serve the vulnerable. WHO's efforts focus on improving health systems, addressing global health challenges, and providing technical assistance to countries.

**Why Each Wrong Option is Incorrect**
**Option A:** is incorrect because it is the correct answer, and there's no need to explain why.
**Option B:** is incorrect because 21st May is not a significant date in the history of WHO. This date might be associated with other global health events or awareness campaigns.
**Option C:** is incorrect because 2nd August is not a notable date in the context of WHO's founding or significant events. This date might be associated with other global health initiatives or awareness campaigns.
**Option D:** is incorrect because 31st December is not a relevant date in the history of WHO. This date might be associated with New Year's Eve celebrations or other global events.

**Clinical Pearl / High-Yield Fact**
WHO Day serves as a reminder of the importance of global health cooperation and the need for collective action to address health challenges. It highlights the significance of WHO's role in shaping global health policies and promoting health awareness worldwide.

**✓ Correct Answer: A. 7th April**