Explanation: The device described, an LED facing a photodiode in a light-tight enclosure, is commonly known as an **optoisolator**. An optoisolator (also called an optocoupler) uses light to transfer an electrical signal between two isolated circuits. The LED converts the electrical input signal into a light signal, which then illuminates the photodiode. The photodiode converts the light back into an electrical signal for the output circuit. The light-tight enclosure ensures that only the light from the internal LED reaches the photodiode, providing complete electrical isolation between the input and output circuits. This is crucial in radio electronics for preventing ground loops, protecting sensitive components from high voltage, and eliminating noise coupling. A seven-segment LED (B) is a display device. An optointerrupter (C) typically has a gap between the LED and photodetector to detect objects breaking the light beam. An infra-red (IR) detector (D) is a broad term for any device sensing IR, not specifically this enclosed pair for isolation.