Assertion : Electron affinity refers to an isolated atom’s attraction for an additional electron while electronegativity is the ability of an element to attract electrons towards itself in a shared pair of electrons.
Reason : Electron affinity is a relative number and electronegativity is experimentally measurable.

Options

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

Explanation

To analyze the given assertion and reason, we will break down each statement and evaluate their accuracy.

### Step 1: Understanding the Assertion
The assertion states: "Electron affinity refers to an isolated atom’s attraction for an additional electron while electronegativity is the ability of an element to attract electrons towards itself in a shared pair of electrons."

- **Electron Affinity**: This is indeed the energy change that occurs when an electron is added to a neutral atom in the gas phase. It reflects how much an atom "wants" to gain an electron.

- **Electronegativity**: This is the tendency of an atom to attract shared electrons when it is part of a compound. It is a measure of the ability of an atom to attract bonding electrons.

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

### Step 2: Understanding the Reason
The reason states: "Electron affinity is a relative number and electronegativity is experimentally measurable."

- **Electron Affinity**: While it can be expressed as a numerical value (usually in kJ/mol), it is not a relative number in the same sense as electronegativity. It can be measured for individual atoms and is not relative to other elements.

- **Electronegativity**: This is indeed a relative measure, often calculated based on various scales (like the Pauling scale). It is not directly measurable in the same way as electron affinity.

**Conclusion for Step 2**: The reason is **false** because it incorrectly categorizes electron affinity as a relative number.

### Final Conclusion
Based on the analysis:
- The assertion is **true**.
- The reason is **false**.

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