Assertion: The standard unit for expressing the mass of atoms is a.m.u.
Reason: a.m.u. stands for mass of 1 atom of carbon.

Options

A. Assertion and reason both are correct statements and reason is correct explanation for assertion.
B. Assertion and reason both are correct statements but reason is not correct explanation for assertion.
C. Assertion is correct statements but reason is wrong statement.
D. Assertion is wrong statement but reason is correct statement.

Answer

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

### Step 1: Analyze the Assertion
The assertion states that "The standard unit for expressing the mass of atoms is a.m.u."

- The term "a.m.u." stands for "atomic mass unit."
- The atomic mass unit (a.m.u.) is indeed the standard unit used to express the mass of atoms.

**Conclusion for Step 1**: The assertion is true.

### Step 2: Analyze the Reason
The reason states that "a.m.u. stands for mass of 1 atom of carbon."

- This statement is misleading. The atomic mass unit (a.m.u.) is defined as 1/12th the mass of a carbon-12 atom, not the mass of a single carbon atom itself.
- Therefore, the reason provided is incorrect.

**Conclusion for Step 2**: The reason is false.

### Step 3: Determine the Relationship
Now we need to determine the relationship between the assertion and the reason.

- The assertion is true, while the reason is false.
- This means that the assertion stands independently of the reason.

### Final Conclusion
Based on the analysis:
- The assertion is correct.
- The reason is incorrect.

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