Assertion : Rate of radioactivity cannot be increased or decreased by increasing or decreasing pressure or temperatur.
Reason : Rate depends on the number of nuclei present in the radioactive sample.

Options

A. (a) If both Assertion and Reason are true and Reason is correct explanation of the Assertion.
B. (b) If both Assertion and Reason are true but Reason is not the correct explanation of Assertion.
C. (c) If Assertion is true, but the Reason is false.
D. (d) If Assertion is false but the Reason is true.

Explanation

To solve the problem, we need to analyze both the assertion and the reason provided in the question.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
The assertion states that the rate of radioactivity cannot be increased or decreased by changing the pressure or temperature. This means that external conditions like pressure and temperature do not influence the rate of radioactive decay.

2. **Understanding the Reason**:
The reason given is that the rate of radioactivity depends on the number of nuclei present in the radioactive sample. This implies that the more nuclei there are, the higher the rate of decay, but it does not mention anything about external conditions affecting this rate.

3. **Analyzing the Relationship**:
In radioactive decay, the rate is defined by the decay constant (λ) and the number of radioactive nuclei (N) present in the sample. The relationship can be expressed as:
\[ \text{Rate of decay} = \lambda N \]
Here, λ is a constant specific to the radioactive material and does not change with pressure or temperature.

4. **Conclusion on Assertion and Reason**:
Since the rate of decay is indeed dependent on the number of nuclei and not on external conditions like pressure or temperature, the assertion is true. The reason is also true, but it does not explain why the assertion holds true. Thus, while both statements are correct, the reason does not provide a valid explanation for the assertion.

5. **Final Answer**:
Both the assertion and reason are true, but the reason is not a correct explanation for the assertion. Therefore, the correct answer is option B.