Assertion : Nuclear energy can be produced only by fusion of certain elements.
Reason : Nuclear reactions generate enormous energy.

Options

A. If both the assertion and the reason are true and the reason is a correct explanation of the assertion
B. If both the assertion and reason are true but the reason is not a correct explanation of the assertion
C. If the assertion is true but the reason is false
D. If the assertion is false but reason is true

Explanation

To analyze the given statements, we need to evaluate both the assertion and the reason provided.

### Step 1: Evaluate the Assertion
The assertion states that "Nuclear energy can be produced only by fusion of certain elements." This statement is incorrect because nuclear energy can be produced not only through fusion but also through fission. Fission is the process of splitting heavy atomic nuclei into lighter nuclei, which also releases a significant amount of energy. Therefore, the assertion is **false**.

### Step 2: Evaluate the Reason
The reason states that "Nuclear reactions generate enormous energy." This statement is correct. Both fusion and fission nuclear reactions release a tremendous amount of energy due to the conversion of mass into energy as described by Einstein's equation \(E=mc^2\). Thus, the reason is **true**.

### Step 3: Conclusion
Based on the evaluations:
- The assertion is **false**.
- The reason is **true**.

Therefore, the correct answer is that the assertion is false but the reason is true.

### Final Answer
Assertion is false, and Reason is true.