**Core Concept**
The eligible couple register is a demographic tool used to track and monitor the reproductive health of couples in a population. It is an essential component of family planning and reproductive health programs, helping to identify couples who are eligible for contraceptive services and other reproductive health interventions.

**Why the Correct Answer is Right**
The eligible couple register is maintained at the sub-centre level, which is the most peripheral and first contact point between the primary health care system and the community. Sub-centres are responsible for providing basic health services, including family planning and reproductive health services, to the surrounding population. The eligible couple register is used to identify couples who are eligible for these services, allowing health workers to target their interventions more effectively.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect, as the eligible couple register is not typically maintained at the village health nurse's level.
**Option B:** Incorrect, as the eligible couple register is not typically maintained at the primary health centre level.
**Option C:** Incorrect, as the eligible couple register is not typically maintained at the community health centre level.

**Clinical Pearl / High-Yield Fact**
It's essential for health workers to maintain accurate and up-to-date eligible couple registers to ensure that couples receive timely and effective reproductive health services.

**Correct Answer:** C. Community health centre