The ability to regulate the amount of charge flowing in a circuit is pennfoster: (a. resistance.) b. direct current. c. alternating current. (d. voltage.)

You can think of resistance as a sort of friction. It can limit the flow of charge in a circuit. Hindering the flow allows it to regulate how much current will flow through the device. Resistance is your answer, letter A. 

If you take into account the voltage, although it can also regulate the current by choosing a power source, when you talk about the charge flowing WITHIN the circuit given a specific power source, it is resistance that will temper the flow. 

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