Bank runs are a problem because banks only hold a fraction of deposits as reserves.
Option B is correct
Explanation:
Fractional reserve banking is a banking system that includes only a percentage of the deposit made by its customers. This enables them to make loans with the remainder and thus effectively create new money. This allows commercial banks to control the money supply directly.
Nevertheless, while central banks control the money supply, commercial banks raise the bulk of the money in contemporary economies by fractional reserve banking.