Explanation: An optoisolator, also known as an optocoupler, is a device that transfers electrical signals between two electrically isolated circuits using light. It fundamentally consists of an LED (Light-Emitting Diode) on the input side, which converts an electrical signal into light. This light then shines on a photosensitive device (such as a phototransistor, photodiode, or photoresistor) on the output side, which converts the light back into an electrical signal. This setup provides crucial galvanic isolation, protecting sensitive control circuitry from noisy or high-voltage power circuits, preventing ground loops, and ensuring signal integrity—a common requirement in amateur radio interfaces and control systems. Option B describes a photodiode or solar cell, which generates charge when exposed to light, but is not the complete isolation component. Options C and D combine unrelated components; a capacitor stores charge, and a battery provides power, neither of which constitutes an optoisolator.