Explanation:

International Development Association (IDA) is called the ‘soft’ window of the World Bank (WB) because its loans are interest free.
The International Development Association (IDA) is an international financial institution which offers concessional loans and grants to the world's poorest developing countries. The IDA is a member of the World Bank Group and is headquartered in Washington, D.C., United States. It was established in 1960 to complement the existing International Bank for Reconstruction and Development by lending to developing countries which suffer from the lowest gross national income, from troubled creditworthiness, or from the lowest per capita income.