Assertion : Atomic mass of Na is 23.
Reason : An atom of sodium is 23 times heavier than 1/12th mass of C-12 isotope.

Options

A. If both Assertion & Reason are true and the reason is the correct explanation of the assertion.
B. If both Assertion & Reason are true but the reason is not the correct explanation of the assertion.
C. If Assertion is true statement but Reason is false.
D. If both Assertion and Reason are false statements.

Answer

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

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that the atomic mass of sodium (Na) is 23.
- This is a factual statement. The atomic mass of sodium is indeed approximately 23 atomic mass units (amu).

2. **Understanding the Reason**:
- The reason states that an atom of sodium is 23 times heavier than 1/12th the mass of the carbon-12 isotope.
- The carbon-12 isotope is defined such that 1/12th of its mass is 1 amu. Therefore, the mass of one carbon-12 atom is 12 amu.
- If we take 1/12th of the mass of carbon-12, it equals 1 amu.
- Thus, saying that an atom of sodium is 23 times heavier than 1/12th the mass of carbon-12 means that the mass of sodium is 23 amu (since 23 * 1 amu = 23 amu).

3. **Conclusion**:
- Both the assertion and the reason are correct.
- The assertion correctly states the atomic mass of sodium, and the reason correctly explains the relationship of sodium's mass to the carbon-12 isotope.

### Final Answer:
- Both the assertion and the reason are correct.