What phenomenon causes electrical current flow in a circuit?

The correct choice refers to potential difference, which is fundamentally crucial for the flow of electrical current in a circuit. Potential difference, often referred to as voltage, is the difference in electric potential energy per unit charge between two points in an electric field. This difference creates a force that drives the electrons (or charge carriers) from the area of higher potential to the area of lower potential, resulting in an electric current.

In essence, without a potential difference, there is no force to move the charge carriers, and thus, no current would flow in the circuit. Voltage must be present to provide the energy needed for the electrons to overcome any resistance within the circuit and continue flowing.

The other phenomena listed—magnetic fields, resistance, and capacitance—play important roles in electrical systems but do not directly cause current flow. Magnetic fields can affect a current, especially in devices like generators, and resistance impedes current flow rather than initiates it. Capacitance refers to the ability of a system to store charge, but it does not inherently create current flow either.