## Core Concept
A sub-centre is the most peripheral and first contact point between the primary healthcare system and the community. It plays a crucial role in delivering basic healthcare services, especially in rural and underserved areas. The sub-centre is an integral part of the healthcare delivery system in developing countries.

## Why the Correct Answer is Right
The correct answer, **5,000 population in plain areas and 3,000 population in hilly, tribal, or backward areas**, reflects the standard norms for the coverage area of a sub-centre. According to the World Health Organization (WHO) and various national health policies, sub-centres are designed to provide accessible healthcare services to a specific population. The population coverage varies based on the terrain and accessibility of the area.

## Why Each Wrong Option is Incorrect
* **Option A:** This option is incorrect because it does not specify the correct population coverage for a sub-centre.
* **Option B:** This option is incorrect as it mentions a larger population, which is typically covered by a Primary Health Centre (PHC), not a sub-centre.
* **Option D:** This option is incorrect because it provides a range that does not accurately reflect the standard population coverage for a sub-centre.

## Clinical Pearl / High-Yield Fact
A key point to remember is that sub-centres are essential for providing basic healthcare services, including maternal and child health services, immunizations, and family planning services. They serve as a critical link in the healthcare delivery system, especially in rural and hard-to-reach areas.

## Correct Answer: C. 5,000 population in plain areas and 3,000 population in hilly, tribal, or backward areas.