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 analyze the given assertion (A) and reason (R), we will break down the statements and provide a step-by-step solution.

### Step-by-Step Solution:

1. **Understanding the 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 the particles emitted during radioactive decay. Alpha decay typically occurs in heavy nuclei, while beta decay occurs in neutron-rich nuclei.

2. **Understanding the 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 refers to the ratio of neutrons (N) to protons (Z) in a nucleus. A stable nucleus has a specific range of N/Z ratios. When this ratio is outside the stable range (specifically when N is much less than Z), the nucleus tends to undergo alpha decay.

3. **Analyzing the Relationship Between A and R**:
- The assertion (A) is indeed correct because alpha and beta emissions are governed by different conditions of nuclear stability.
- The reason (R) is also correct, as it accurately describes a condition under which alpha decay occurs.
- However, while both statements are true, the reason does not directly explain why alpha and beta particles cannot be emitted simultaneously. They are governed by different decay processes that typically occur under different conditions.

4. **Conclusion**:
- Since both the assertion and the reason are correct but the reason does not adequately explain the assertion, we conclude that:
- Assertion (A) is true.
- Reason (R) is true, but it does not correctly explain (A).

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