**Core Concept**
The establishment of Primary Health Centres (PHCs) is based on the population norms, which vary according to the area type, such as plain, hilly, or tribal areas. In **tribal areas**, the population norms for establishing PHCs are crucial for ensuring accessible healthcare.

**Why the Correct Answer is Right**
In **tribal areas**, one PHC is established per **20,000** population, considering the difficult terrain and dispersed population. This norm helps in providing basic medical care and preventive healthcare services to the tribal population. The **National Health Mission** and other healthcare policies emphasize the importance of PHCs in providing comprehensive healthcare.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it does not align with the population norm for tribal areas.
**Option B:** This option is also incorrect as the population norm for plain areas is different from that of tribal areas.
**Option D:** This option is incorrect as it exceeds the recommended population norm for establishing a PHC in tribal areas.

**Clinical Pearl / High-Yield Fact**
Remembering the population norms for PHCs in different areas is essential for healthcare planning and policy-making. In **tribal areas**, the focus is on providing accessible healthcare due to the challenging terrain.

**Correct Answer:** C. 20,000 population