Step 1: The question targets the bottom-most rung of the rural health infrastructure. That rung is the Sub-Centre, which is the closest health facility to the community.
Step 2: Functionally, the Sub-Centre is the front line for behaviour-change communication and for primary services such as MCH care, family welfare, nutrition advice, immunization, and diarrhoea and communicable-disease control.
Step 3: It also keeps a small stock of essential drugs for minor ailments. As the most peripheral first-contact unit, it outranks the PHC and Block centre in remoteness, and the Anganwadi belongs to ICDS, not the health-centre chain.
\[\boxed{\text{Sub-centre}}\]