**Core Concept**
The National Mental Health Programme (NMHP) is a government initiative to provide mental health services to the population. The programme aims to integrate mental health into general health services and to reduce the stigma associated with mental illness. The implementation of NMHP requires a structured approach, with a focus on community-based care.

**Why the Correct Answer is Right**
The functional unit for implementation of NMHP is at the district level. This is because the district is the most appropriate level for implementing community-based mental health services. The district level provides a balance between the availability of resources and the need for accessibility to mental health services. At this level, mental health services can be integrated into existing primary health care services, making it easier for people to access care.

**Why Each Wrong Option is Incorrect**
* **Option A:** The state level is too broad for implementing NMHP. While the state government plays a crucial role in planning and coordinating mental health services, the day-to-day implementation of the programme requires a more localized approach.
* **Option B:** The sub-district level is not a recognized administrative level in most countries. While this level may be useful for planning and coordination, it is not a feasible option for implementing NMHP.
* **Option C:** The community level is too broad and vague. While community-based care is a key component of NMHP, the programme requires a more structured approach, with clear lines of authority and accountability.

**Clinical Pearl / High-Yield Fact**
The district level is the most appropriate level for implementing NMHP because it allows for a balance between resource availability and accessibility to mental health services.

**Correct Answer:** C. District level