Which of the following commands a contactor to open or close?

The control coil is the component responsible for commanding a contactor to open or close. When an electric current flows through the control coil, it generates a magnetic field that attracts or repels the armature of the contactor. This mechanical movement either makes or breaks the connection between the contacts, thus controlling the circuit connected to the contactor.

Current sensors, mechanical switches, and power supplies serve different functions in an electrical system. Current sensors monitor the current flowing through a circuit but do not directly control the contactor. Mechanical switches can control circuits but are not part of the contactor mechanism itself. A power supply provides the necessary voltage and current to the system but does not inherently have the mechanism to open or close the contactor on its own. Therefore, the control coil is the essential element that specifically performs the function of commanding the contactor to engage or disengage.