Assertion : One atomic mass unit (amu) is mass of an atom equal to exactly one-twelfth of the mass of a carbon-12 atom.
Reason : Carbon-12 isotope was selected as standard.

Options

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

Answer

To analyze the assertion and reason provided in the question, we can break it down into steps:

### Step 1: Understanding Atomic Mass Unit (amu)
- The atomic mass unit (amu) is a standard unit of mass that quantifies mass on an atomic or molecular scale.
- It is defined such that one amu is equal to one-twelfth of the mass of a carbon-12 atom.

### Step 2: Definition of Carbon-12
- Carbon-12 is a stable isotope of carbon with 6 protons and 6 neutrons, making it a common reference point for atomic mass.
- The choice of carbon-12 as a standard allows for consistent measurements across different elements.

### Step 3: The Assertion
- The assertion states that one atomic mass unit (amu) is defined as exactly one-twelfth of the mass of a carbon-12 atom.
- This is a true statement as it is the basis for the definition of amu.

### Step 4: The Reason
- The reason provided states that the carbon-12 isotope was selected as a standard for measuring atomic mass.
- This is also true, as carbon-12 was chosen due to its abundance and stability, making it a reliable reference.

### Step 5: Conclusion
- Both the assertion and the reason are true, and the reason correctly explains why the assertion is true.

### Final Answer
- Therefore, the assertion is true, and the reason is also true, and the reason supports the assertion.

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