Through which tool does the federal reserve affect money available for banks to loan?

Reserve requirement, is the right answer.

Explanation:

A regulation of the central bank used by most but not all of the central banks of the world, that places the minimum volume of reserves a commercial bank must be held, is known as the reserve requirement. The minimum reserve is usually prepared by the central bank to be no limited than a particularized % of the amount of collateral liabilities the commercial bank owes to its clients.

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