Assertion(A) One atomic mass unit is defined as one twelth of the mass of one carbon-12 atom.
Reason(R) Carbon-12 isotope is the most abundant isotope of carbon and has been chosen as standard.

Options

A. Both A and R are true and R is the correct explanation of A.
B. A is false but R is true
C. A is true but R is false.
D. Both A and R are false.

Explanation

### Step-by-Step Solution:

1. **Understanding the Assertion:**
- The assertion states that "one atomic mass unit (amu) is defined as one twelfth of the mass of one carbon-12 atom."
- This is a standard definition in chemistry used to express atomic masses.

2. **Understanding the Reason:**
- The reason given is that "carbon-12 isotope is the most abundant isotope of carbon and has been chosen as standard."
- This statement is true; carbon-12 is indeed the most stable and abundant isotope of carbon.

3. **Evaluating the Truth of the Assertion:**
- The assertion is true because the definition of atomic mass unit is based on the carbon-12 isotope.
- By definition, 1 amu = 1/12 the mass of a carbon-12 atom.

4. **Evaluating the Truth of the Reason:**
- The reason is also true as carbon-12 is the most abundant isotope of carbon.
- However, the reason provided does not explain why carbon-12 was chosen as the standard for atomic mass units.

5. **Identifying the Correct Explanation:**
- The correct explanation for the assertion should focus on the comparison of the masses of different elements to the carbon-12 atom.
- Scientists found that using carbon-12 as a standard allowed for most atomic masses to be expressed as whole numbers, simplifying calculations.

6. **Conclusion:**
- Both the assertion and reason are true, but the reason does not correctly explain the assertion.
- Therefore, the correct answer is: "Both assertion and reason are true, but R is not the correct explanation of A."

### Final Answer:
The correct option is: "Both assertion and reason are true, but R is not the correct explanation of A."

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