**Core Concept**

The question is asking about the purpose or function of a sub-centre in a medical context, specifically regarding the population it serves. In this case, a "sub-centre" is typically a primary healthcare facility in India's public healthcare system. Sub-centres are a part of the Primary Health Centre (PHC) network and play a crucial role in providing basic medical services to the population.

**Why the Correct Answer is Right:**

The correct answer, **D**, refers to a "Sub-Centre" serving a "Village". In the Indian National Health System, a sub-centre is a primary health care unit that caters to the health needs of a village or a group of villages. These sub-centres are part of Primary Health Centre (PHC) network and provide basic medical services to the population living in their designated catchment area.

**Why Each Wrong Option is Incorrect:**

A) A "Health Centre" (option A) serves a larger population than a sub-centre, typically a cluster of villages or a small town. They are more advanced than sub-centres and can provide specialist services.

B) A "Village Health & Family Welfare Centre" (option B) serves a similar population as a health centre, focusing on both preventive and curative services. However, it is not the correct answer as it is a higher level facility than a sub-centre.

C) A "Community Health Centre" (option C) is a higher level facility in the Indian healthcare system, serving a larger population than a sub-centre and including specialist services.

**Clinical Pearl / High-Yield Fact:**

Understanding the hierarchy of healthcare facilities in India's National Health System is essential for students preparing for medical exams. Each level of the healthcare system is designed to cater to a specific population size and provides a range of services tailored to meet their needs. This ensures a continuum of care for patients, from primary to tertiary care. Familiarize yourself with the hierarchy to effectively navigate the Indian healthcare system and provide appropriate healthcare recommendations to patients based on the facility they visit.