Assertion (A): The standard unit of expressing the mass of atom is amu.
Reason (R ): amu is also called as avogram.

Options

A. If both (A) and (R ) are correct and (R ) is the correct explantion for (A)
B. If both (A) and (R ) are correct but (R ) is not the correct explantion for (A)
C. If (A) is correct but (R ) is incorrect.
D. If (A) and (R ) are incorrect.

Explanation

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

### Step-by-Step Solution:

1. **Understanding the Assertion (A)**:
- The assertion states that "The standard unit of expressing the mass of an atom is amu."
- This is correct. The atomic mass unit (amu) is indeed the standard unit used to express the mass of atoms and molecules. It is defined as one twelfth (1/12) of the mass of a carbon-12 atom.

2. **Understanding the Reason (R)**:
- The reason states that "amu is also called avogram."
- This statement is incorrect. The term "avogram" is not synonymous with amu. Instead, the term "avogram" refers to a unit of mass that is related to Avogadro's number, which is used in the context of moles and molecular mass. The correct term associated with Avogadro's number is "gram per mole," not "avogram."

3. **Conclusion**:
- Since the assertion is correct but the reason is incorrect, we conclude that the assertion is true, while the reason does not provide a valid explanation for the assertion.

4. **Final Answer**:
- The correct option for this question is that the assertion is true, but the reason is false.