**Core Concept**
World Tuberculosis Day is a global health awareness day that commemorates the discovery of Mycobacterium tuberculosis, the bacteria responsible for tuberculosis (TB), by Robert Koch in 1882. This day aims to raise awareness about the devastating impact of TB and promote efforts to control and eliminate the disease.

**Why the Correct Answer is Right**
TB is a major public health concern worldwide, causing millions of deaths annually. The discovery of M. tuberculosis by Koch marked a significant milestone in the understanding of the disease, leading to the development of diagnostic tools, treatments, and control measures. The World Health Organization (WHO) and other global health authorities observe World TB Day on March 24th to highlight the need for continued efforts to combat TB.

**Why Each Wrong Option is Incorrect**
**Option A:** This is incorrect because World TB Day is not celebrated on April 1st, which is known for being April Fool's Day.
**Option B:** This is incorrect because World TB Day is not observed on June 21st, which is the summer solstice.
**Option C:** This is incorrect because World TB Day is not celebrated on December 7th, which is a date associated with a different global health awareness campaign.

**Clinical Pearl / High-Yield Fact**
TB is a leading cause of death worldwide, particularly among people living with HIV/AIDS. The "DOTS" (Directly Observed Treatment, Short-Course) strategy, introduced in the 1990s, has been instrumental in reducing the incidence of multidrug-resistant TB.

**Correct Answer:** C. March 24th.