A : During radioactive disintegration an `alpha`-particle and a `beta`-particle do not emit simultaneously from any nucleus.
R : An `alpha`-particle emits from a nucleus when the N/Z ratio is less than the stability range (where N=number of neutrons and Z=number of protons in a nucleus. )

Options

A. if both assertion & reason are true and the reason is correct explanation of the assertion then mark (1)
B. if both assertion & reason are true and the reason is not correct explanation of the assertion then mark (2)
C. if assertion is true statement but reason is false then mark (3)
D. if both assertion & reason are false then mark (4)

Answer

To solve the question, we need to analyze both the assertion (A) and the reason (R) provided:

### Step-by-Step Solution:

1. **Understanding Assertion (A)**:
- The assertion states that during radioactive disintegration, an alpha particle and a beta particle do not emit simultaneously from any nucleus.
- This is based on the nature of radioactive decay. Alpha decay and beta decay are two distinct processes that occur under different conditions. Typically, a nucleus will undergo one type of decay at a time to achieve stability.

2. **Understanding Reason (R)**:
- The reason states that an alpha particle emits from a nucleus when the N/Z ratio is less than the stability range.
- The N/Z ratio (neutrons to protons ratio) is crucial for nuclear stability. Nuclei with a low N/Z ratio tend to undergo alpha decay to reduce their mass and achieve a more stable configuration.

3. **Evaluating the Truth of A and R**:
- Both statements are true. The assertion is correct because alpha and beta emissions are mutually exclusive processes in a single decay event. The reason is also correct as it describes the condition under which alpha decay occurs.

4. **Determining the Relationship**:
- While both A and R are true, the reason does not provide a correct explanation for the assertion. The assertion is about the simultaneous emission of alpha and beta particles, while the reason discusses the conditions for alpha decay specifically.

5. **Conclusion**:
- Therefore, we conclude that both the assertion (A) and the reason (R) are true, but R is not the correct explanation for A.

### Final Answer:
- Assertion (A) is true.
- Reason (R) is true, but it does not explain A.