**Core Concept**
The Revised National Tuberculosis Control Programme (RNTCP) is a **public health initiative** aimed at controlling and preventing tuberculosis in India. It involves the establishment of **tubercular units** to provide effective healthcare services.

**Why the Correct Answer is Right**
The RNTCP guidelines specify that one **tubercular unit** should cover a population of 500,000. This is based on the principle of providing accessible and equitable healthcare services, ensuring that the population has access to **diagnostic and treatment facilities** for tuberculosis.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not align with the RNTCP guidelines.
**Option B:** This option is also incorrect as it exceeds the recommended population coverage.
**Option D:** This option is incorrect because it falls short of the recommended population coverage.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that the RNTCP aims to provide **universal access** to tuberculosis diagnosis and treatment. The population coverage by one tubercular unit is a critical aspect of this initiative.

**Correct Answer:** C. 500,000