Explanation:

- Regional Rural Bank (Option 1): These banks are local level banking organizations operating in different states. They provide banking services to rural sectors but are not the apex agency for rural credit.

- NABARD (Option 2): Stands for National Bank for Agriculture and Rural Development. It is the apex institution tasked with facilitating credit flow for the promotion and development of agriculture and rural sectors.

- SBI (Option 3): The State Bank of India is a large public sector bank that offers services across India but does not specifically hold the apex role in rural credit.

- None of these (Option 4): Incorrect as NABARD is indeed the apex body for rural credit.