*Question E8C09:
Which digital code allows only one bit to change between sequential code values?*  

*Correct Answer is  D: Gray code*  

### AI Response: (may contain errors)  

Gray code is the correct answer because it is specifically designed so that only one bit changes between consecutive values. This property is useful in minimizing errors in digital systems, such as encoders, where transitioning between states can cause glitches if multiple bits change simultaneously.

### Explanation:

1. **Gray Code**:
   - **Correct Answer**: Gray code is a binary numeral system where two successive values differ in only one bit. This is useful in error reduction in digital systems, especially in analog-to-digital conversion and rotary encoders.

2. **Other Choices**:
   - **Binary Coded Decimal Code (BCD)**:
     - BCD represents each decimal digit with its binary equivalent. For example, the decimal number 5 is represented as 0101 in BCD. Multiple bits can change between consecutive numbers, so it does not have the one-bit change property.
   - **Extended Binary Coded Decimal Interchange Code (EBCDIC)**:
     - EBCDIC is an 8-bit character encoding used mainly on IBM mainframes. It is not designed to have the one-bit change property between sequential values.
   - **Extended ASCII**:
     - Extended ASCII is an 8-bit character encoding that extends the standard ASCII set. Like EBCDIC, it is not designed for the one-bit change property.

### Key Point:
- **Gray Code**: Only one bit changes between sequential values, minimizing errors in digital transitions. This is why it is the correct answer for the question.