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

A light-emitting diode (LED) is commonly used as a visual indicator due to its ability to emit light when an electric current passes through it. LEDs are highly efficient and can produce bright light in a variety of colors, making them suitable for signaling and display purposes in many electronic devices and applications. Their simplicity in operation ensures that they can effectively indicate on/off states or other status information, which is why they are prevalent in indicator lights on devices, traffic signals, and various consumer electronics.

Other components, such as field-effect transistors (FETs), Zener diodes, and bipolar transistors, serve different roles in electronic circuits. While Zener diodes work in voltage regulation, FETs and bipolar transistors are primarily used for amplification and switching, rather than providing visual feedback. Thus, these components do not serve the purpose of a visual indicator in the same direct and effective manner as an LED does.