The code where all successive numbers differ from their preceding number by single bit is  *(A) Binary code.(B) BCD.(C) Excess – 3.(D) Gray

To solve the problem of identifying the code where all successive numbers differ from their preceding number by a single bit, let's analyze the choices provided:

1. Binary Code: This does not necessarily ensure that successive numbers differ by a single bit. For example, from 3 (011) to 4 (100), there are multiple bit changes.

2. BCD (Binary-Coded Decimal): BCD represents decimal numbers and does not maintain the single bit change property.

3. Excess-3: This is a non-weighted code used to express decimal numbers, but it also does not guarantee a single bit difference between successive codes.

4. Gray Code: This is a binary numeral system where two successive values differ in only one bit. This property makes it unique for applications in digital systems, particularly in error correction and rotary encoders.

Final Answer

The correct answer is (D) Gray.