**Core Concept**
The Reproductive and Child Health (RCH) program is a government initiative aimed at improving maternal and child health outcomes. The program involves various levels of healthcare delivery, from sub-centres to district hospitals. Effective planning and implementation of RCH services require a well-structured approach, with a focus on the peripheral most unit that can reach the most vulnerable populations.

**Why the Correct Answer is Right**
The sub-centre is the peripheral most unit in the public health system, responsible for providing basic healthcare services to a population of 5,000 in the rural areas and 3,000 in the urban areas. It is the first point of contact between the community and the healthcare system, providing essential services such as maternal and child healthcare, family planning, and immunization. The sub-centre is staffed by an auxiliary nurse midwife (ANM) and a male multipurpose health worker (MHW). They are responsible for conducting health check-ups, providing health education, and referring patients to higher-level facilities when needed.

**Why Each Wrong Option is Incorrect**
**Option B:** The block or taluka is a higher-level administrative unit that oversees multiple sub-centres and primary health centres (PHCs). While it plays a crucial role in planning and implementing RCH services, it is not the peripheral most unit.

**Option C:** The primary health centre (PHC) is a higher-level facility that provides basic medical care, including family planning and maternal healthcare services. However, it is not the peripheral most unit, as it serves a larger population and has more advanced facilities than a sub-centre.

**Option D:** The district is the highest level of administrative unit, responsible for overseeing multiple blocks and PHCs. While it plays a crucial role in planning and implementing RCH services, it is not the peripheral most unit, as it serves a large population and has more advanced facilities than a sub-centre.

**Clinical Pearl / High-Yield Fact**
The sub-centre is the most peripheral unit in the public health system, and its effectiveness in delivering RCH services depends on the availability of trained staff, adequate infrastructure, and community engagement. Ensuring the sub-centre is equipped to provide quality services is critical for improving maternal and child health outcomes.

**✓ Correct Answer: A. Sub-centre**