Which of the following is commonly used as a visual indicator?

An LED, or Light Emitting Diode, is commonly used as a visual indicator because it emits light when an electric current flows through it. This characteristic makes LEDs ideal for providing visual feedback in a variety of electronic circuits and devices, such as indicators on radios, appliances, and other consumer electronics.

The functionality of an LED allows it to indicate whether a device is powered on, performing a specific function, or showing an error condition. Its wide availability, low power consumption, and variety of colors further contribute to its popularity as a visual indicator.

In contrast, a FET (Field Effect Transistor) is primarily used for switching and amplifying electronic signals rather than for providing visual feedback. A Zener diode serves specific functions in voltage regulation and protection, and while it can indicate voltage conditions indirectly, it does not serve as a visual indicator in the same way that an LED does. Similarly, a bipolar transistor is mainly used for amplification and switching tasks without offering a visual cue on its own.