D – Cooperative Society
Formal sources of credit include banks and cooperative societies, supervised by the Reserve Bank of India. Relatives, traders, and moneylenders are informal sources.
Source: Money and Credit, Formal Sector Credit in India
Examiners look for the correct option with a brief justification. Remember: formal sources = banks + cooperatives (regulated by RBI); informal sources = moneylenders, traders, employers, relatives, friends. Trader (B) is explicitly listed as an informal lender in the textbook, so don't confuse it with a formal source.