Explanation: An optocoupler (or opto-isolator) is an electronic component that transfers electrical signals between two isolated circuits using light. It typically contains an LED (light-emitting diode) on the input side and a light-sensitive receiver, like a phototransistor or photodiode, on the output side, all housed within a single package. The light emitted by the LED is detected by the receiver, converting the optical signal back into an electrical one. This arrangement provides excellent electrical isolation, preventing high voltages or noise from one circuit from affecting the other, which is crucial in many radio and electronic designs for protection and noise reduction. Options A, B, and D are incorrect: A) **A resistor and a capacitor** are basic passive components and do not describe an optocoupler. B) **Two light sources modulated onto a mirrored surface** describes a different type of optical sensing arrangement, not an optocoupler's function of signal transfer with isolation. D) **An amplitude modulated beam encoder** is a specific type of optical encoder, not a general description of an optocoupler's purpose or construction.