(A) Both (A) and (R) are true and (R) is the correct explanation of (A).
Rural poor lack collateral and proper documents required for bank loans, so they are forced to depend on informal sources of credit.
The passage from "Self-Help Groups for the Poor" directly states: "bank loans require proper documents and collateral. Absence of collateral is one of the major reasons which prevents the poor from getting bank loans" — this is exactly why they depend on informal sources. So (R) correctly and directly explains (A), making option (A) the right choice.