Which of the following process represents a `gamma- decay`?

Options

A. `.^A X_Z +r to .^A X_(Z-1)+a+b`
B. `.^AX_Z+.^1n_0 to A-3X_(Z-2)+c`
C. `.^AX_Zto .^AX_Z+f`
D. `.^AX_Z+e_(-1) to .^AX_(Z-1) +g`

Answer

To determine which of the given processes represents gamma decay, we need to understand what gamma decay is and how it differs from other types of radioactive decay.

### Step-by-Step Solution:

1. **Understanding Gamma Decay**:
- Gamma decay is a type of radioactive decay where an unstable nucleus releases energy in the form of gamma radiation (high-energy photons).
- Importantly, gamma decay does not change the atomic number (Z) or the mass number (A) of the nucleus. It only transitions the nucleus from a higher energy state to a lower energy state.

2. **Analyzing the Options**:
- We need to look at the processes provided in the options and identify which one involves the emission of gamma radiation without changing the atomic or mass number.
- For example, if a nucleus undergoes beta decay, it will change into a different element (the atomic number changes), which is not gamma decay.

3. **Identifying the Correct Process**:
- From the information provided in the video transcript, it is mentioned that after a beta decay, the resulting lead nucleus can transition to a lower energy state by emitting gamma radiation.
- In this transition, the atomic mass (A) and atomic number (Z) of the lead nucleus remain unchanged.

4. **Conclusion**:
- Based on the analysis, the option that shows a process where a nucleus emits gamma radiation without changing its atomic mass or atomic number is the correct representation of gamma decay.
- According to the transcript, the third option is the only one that represents gamma decay.

### Final Answer:
The third option represents gamma decay, as it shows a process where the atomic mass and atomic number remain unchanged.