*Question T6D07:
Which of the following is commonly used as a visual indicator?*  

*Correct Answer is  A: LED*  

### AI Response: (may contain errors)  

The correct answer to the question is **A: LED**.

### Explanation:

1. **Why the correct answer is correct:**
   - An **LED (Light Emitting Diode)** is a semiconductor device that emits light when an electric current passes through it. It is commonly used as a visual indicator in various electronic devices, such as indicators on radios, appliances, and other equipment. The light emitted by the LED provides a clear visual cue to the user.

2. **Why the other choices are incorrect:**
   - **B: FET (Field Effect Transistor)**: A FET is a type of transistor used for switching and amplifying electronic signals. It does not emit light and is not used as a visual indicator.
   - **C: Zener diode**: A Zener diode is used primarily for voltage regulation and does not provide any visual indication. It operates in reverse bias to maintain a constant voltage.
   - **D: Bipolar transistor**: A bipolar transistor is also used for switching and amplification. Like the FET, it does not emit light and is not used as a visual indicator.

### Useful Information:
- **LED Characteristics**: 
  - Forward voltage drop typically ranges from 1.8V to 3.3V depending on the color and type.
  - Current limiting resistors are often used in series with LEDs to prevent excessive current flow.

In summary, the LED is the only component among the choices that serves as a visual indicator, making it the correct answer.