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 E`.

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 the reason provided.

### Step 1: Analyze 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 refers to the energy change when an electron is added to an atom in the gaseous state. A more negative value indicates a greater tendency to gain an electron.
- **Fluorine (F)**: Fluorine has the highest electron gain enthalpy because it is one electron short of a stable noble gas configuration (Neon).
- **Oxygen (O)**: Oxygen has a less negative electron gain enthalpy than fluorine because adding an electron to oxygen does not lead to a completely filled shell, but it is still more favorable than nitrogen.
- **Nitrogen (N)**: Nitrogen has a half-filled p subshell (2p3), which is relatively stable. Therefore, it has the least tendency to gain an electron, resulting in the least negative electron gain enthalpy.

Thus, the order of electron gain enthalpy is:
\[ \text{Electron Gain Enthalpy: } \text{F} > \text{O} > \text{N} \]

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

- **Ionization Enthalpy**: This is the energy required to remove an electron from an atom in the gaseous state. A higher ionization enthalpy indicates that it is harder to remove an electron.
- The order of ionization enthalpy is actually \( \text{N} > \text{O} > \text{F} \) due to the stability of the half-filled configuration in nitrogen, which is more stable than the configurations of oxygen and fluorine.

### Conclusion
1. The assertion is true: the magnitude of electron gain enthalpy of oxygen is indeed less than that of fluorine but greater than that of nitrogen.
2. The reason is also true, but it does not correctly explain the assertion because it relates to ionization enthalpy, not electron gain enthalpy.

### Final Answer
Both the assertion and reason are true, but the reason is not a correct explanation of the assertion. Therefore, the correct option is that both statements are true, but the reason is not the correct explanation for the assertion.