Assertion: magnitude of electron gain enthalpy of oxygen is less than that of fluorine but greater than that of nitrogen.
Reason: Ionisation enthalpy order is as follows: `N gt O lt F`.

Options

A. If both assertion and reason are true annd the 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 the reason is false
D. if assertion is false but the reason is true

Answer

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

### Step 1: Understand the Assertion
The assertion states that the magnitude of electron gain enthalpy of oxygen is less than that of fluorine but greater than that of nitrogen.

- **Electron Gain Enthalpy**: This is the energy change when an electron is added to a neutral atom in the gas phase. A more negative value indicates a greater tendency to gain an electron.

#### Evaluation of Assertion:
1. **Fluorine**: Has the highest electron gain enthalpy because it is highly electronegative and has a small atomic size, making it more favorable to gain an electron.
2. **Oxygen**: Has a relatively high electron gain enthalpy, but it is less than that of fluorine due to its larger atomic size and the presence of electron-electron repulsion in its valence shell.
3. **Nitrogen**: Has the least electron gain enthalpy because it has a half-filled p subshell configuration (2p3), which is stable and does not favor gaining an additional electron.

Thus, the assertion is **true**:
- Electron gain enthalpy order: \( \text{E}_a(\text{F}) > \text{E}_a(\text{O}) > \text{E}_a(\text{N}) \).

### Step 2: Understand the Reason
The reason states that the ionization enthalpy order is \( \text{N} > \text{O} > \text{F} \).

- **Ionization Enthalpy**: This is the energy required to remove the most loosely bound electron from an atom.

#### Evaluation of Reason:
1. **Fluorine**: Has the highest ionization enthalpy due to its small size and high electronegativity.
2. **Oxygen**: Has a lower ionization enthalpy than fluorine but higher than nitrogen.
3. **Nitrogen**: Has a half-filled stable electronic configuration, which makes it harder to remove an electron, resulting in a higher ionization enthalpy than expected.

The correct order for ionization enthalpy should actually be \( \text{F} > \text{O} > \text{N} \), not \( \text{N} > \text{O} > \text{F} \). Thus, the reason is **false**.

### Conclusion
- The assertion is true.
- The reason is false.

### Final Answer
Both assertion and reason statements are true, but the reason does not correctly explain the assertion. Therefore, the correct option is:
**Assertion is true, Reason is false.**