What defines a circuit that controls another circuit through a relay or equivalent device?

The correct answer is a control circuit. This type of circuit is specifically designed to control the operation of another circuit, utilizing devices such as relays, contactors, or solid-state devices to perform this function. The main aspect of the control circuit is that it receives an input signal, which activates the control component and, in turn, allows or interrupts the power flow to the main circuit.

Control circuits are critical in automation and safety applications, where precise control and monitoring are necessary. They typically operate at lower voltages and currents, which allows them to safely manage larger loads present in the main circuit without risking direct contact with high voltages.

Other options, like the main circuit, refer to the primary pathway that carries electrical power to the load, while feedback circuits involve returning a portion of the output to the input for regulation purposes, ensuring stability or tracking performance. Remote control circuits might imply operation over a distance but do not specifically highlight the controlling function in relation to another circuit. Thus, the control circuit distinctly identifies the role of managing the operations of another circuit, making it the most appropriate choice.