**Core Concept**
A subcentre is the most peripheral and first contact point between the primary healthcare system and the community. It plays a crucial role in providing basic healthcare services to rural and underserved populations, especially in hilly and tribal areas where accessibility is limited.

**Why the Correct Answer is Right**
In hilly and tribal areas, the population density is low, and geographical terrain can make it challenging to establish a primary health centre (PHC). Therefore, a subcentre is designed to cater to a smaller population, typically in the range of 5,000 to 7,500 individuals. This allows for more efficient healthcare delivery and better outreach services in these hard-to-reach areas.

**Why Each Wrong Option is Incorrect**
* **Option A:** This is incorrect because a subcentre typically caters to a population of 5,000 to 7,500 individuals, not the entire village or town.
* **Option B:** This is incorrect because a subcentre is not specifically designed to cater to urban areas, where population density is higher and healthcare needs are different.
* **Option C:** This is incorrect because a PHC is a more comprehensive healthcare facility that caters to a larger population, typically in the range of 20,000 to 30,000 individuals.

**Clinical Pearl / High-Yield Fact**
In rural and underserved areas, a subcentre can serve as a vital link between the community and the primary healthcare system, providing essential healthcare services, including maternal and child health, family planning, and disease prevention.

**Correct Answer:** D.