## Core Concept
A sub-center is the most peripheral and first contact point between the primary healthcare system and the community. It is a crucial part of the healthcare infrastructure in rural and underserved areas, providing basic medical and health services.

## Why the Correct Answer is Right
The correct answer, **5000 population**, is based on the general guidelines for the population covered by a sub-center. Typically, a sub-center is designed to serve a population of about 5,000 in plain areas and 3,000 in hilly, tribal, or backward areas. This setup is part of the World Health Organization's (WHO) and various national health policies' recommendations to ensure accessible primary healthcare.

## Why Each Wrong Option is Incorrect
* **Option A: 1000 population** - This is too low for the population served by a sub-center. Such a small population would not justify the resources allocated to a full-fledged sub-center.
* **Option B: 30000 population** - This is significantly higher than the population typically served by a sub-center. A population of 30,000 would usually be served by a Primary Health Centre (PHC).
* **Option D: 100000 population** - This is far beyond the coverage of a sub-center, aligning more with the population served by a Community Health Centre (CHC) or a larger healthcare facility.

## Clinical Pearl / High-Yield Fact
A key point to remember is that sub-centers are essential for delivering primary healthcare services, including maternal and child health services, family planning, immunization, and basic curative services. They act as the first line of defense in the healthcare system, especially in rural and underserved areas.

## Correct Answer: C. 5000 population