Answer: The reserve ratio
Explanation: The money multiplier of a commercial bank is the reciprocal of it's reserve ratio. The multiplier helps commercial banks to know how they can increase money supply. The reserve ratio is used to name a certain percentage of a commercial banks deposit which the central bank mandates it to keep as reserve.
Thus:
Multiplier = 1 / reserve ratio
For instance, if reserve ratio is 10%
Multiplier = 1 / 0.1
Multiplier = 10
Therefore, in this scenario, a commercial bank can increase its spending supply by a multiple of 10.