Assertion : Only those nuclei which are heavier than lead are radioactive.
Reason : Nuclei of elements heavier than lead are unstable.

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 analyze the given assertion and reason, we will break down the statements and evaluate their validity step by step.

### Step 1: Evaluate the Assertion
The assertion states: "Only those nuclei which are heavier than lead are radioactive."

- **Analysis**: This statement is incorrect. Radioactivity is not limited to heavy nuclei. In fact, there are several lighter elements (such as Carbon-14, Tritium, etc.) that are also radioactive. Therefore, the assertion is false.

### Step 2: Evaluate the Reason
The reason states: "Nuclei of elements heavier than lead are unstable."

- **Analysis**: This statement is generally true. Elements heavier than lead (atomic number 82) tend to be unstable and are more likely to undergo radioactive decay. This is due to the increasing number of protons and neutrons in the nucleus, which can lead to instability. Therefore, the reason is true.

### Step 3: Conclusion
Since the assertion is false and the reason is true, we can conclude that the correct relationship between the assertion and reason is that the assertion does not support the reason.

### Final Answer
- Assertion: False
- Reason: True