An LED facing a photodiode in a light-tight enclosure is commonly known as a/an:

An LED facing a photodiode in a light-tight enclosure is referred to as an optoisolator because it serves the purpose of transferring electrical signals while providing electrical isolation between its input (the LED) and output (the photodiode).

In this configuration, the LED emits light when an electrical signal is applied to it. This light then strikes the photodiode, which generates a corresponding electrical signal. The photodiode effectively detects the light produced by the LED, allowing signals to be transferred without a direct electrical connection, which enhances safety and protects sensitive electronics from high voltages or surges.

The term "optoisolator" captures this dual functionality: it relates to both the optical (light) connection initiated by the LED and the isolation provided to the circuit, making it an essential component in various applications like switching, signal processing, and data transmission in electronics.

Other options, such as transistor, capacitor, and amplifier, refer to different electronic components or functions that do not specifically address the characteristics and operational principles of an LED and photodiode configuration used for signal isolation.