"Soft loans" = concessional/interest-free loans → always link with IDA, a wing of the World Bank.
Step 1: The International Development Association (IDA) is part of the World Bank Group.
Step 2: IDA provides interest-free or concessional loans (called soft loans) and grants to the world's poorest countries to support development projects.
Step 3: Other agencies mentioned:
- AIIB and ADB provide infrastructure loans, often at commercial rates.
- OPEC focuses on petroleum policies, not soft loans. \[ \boxed{\text{International Development Association (IDA)}} \]
Comparing each organisation's typical lending terms against the idea of "soft loans" clarifies the answer.
The concessional, low-interest nature of its lending is what makes option 4 the right choice.
Hence, the correct answer is International Development Association (IDA).