**Core Concept**
A subcentre in a hilly area is a primary healthcare unit that provides basic medical care to a defined population. The population size that a subcentre caters to is determined by the Indian government's health policy, which takes into account the geographical and demographic characteristics of the area.

**Why the Correct Answer is Right**
In hilly areas, the population density is generally lower due to the challenging terrain and accessibility issues. As a result, the population size that a subcentre caters to is increased to ensure that the healthcare services reach the remote and underserved communities. According to the Indian government's health policy, a subcentre in a hilly area caters to a population of 3000, which is higher than the standard population size of 1000 in plain areas.

**Why Each Wrong Option is Incorrect**
* **Option A:** 1000 is the standard population size for a subcentre in plain areas, not hilly areas.
* **Option B:** 2000 is not the recommended population size for a subcentre in a hilly area.
* **Option D:** 5000 is too large for a subcentre to cater to, even in hilly areas.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the population size that a subcentre caters to varies depending on the geographical location, and hilly areas require a higher population size due to the challenges in accessibility and terrain.

**✓ Correct Answer: C. 3000**