The order of electron gain enthalpy (magnitude) of `O, S` and `Se` is:

Options

A. `O gt S gt Se`
B. `S gt Se gt O`
C. `Se gt S gt O`
D. `S gt O gt Se`

Explanation

To determine the order of electron gain enthalpy (magnitude) of O, S, and Se, we need to understand the concept of electron gain enthalpy and how it varies across periods and down groups in the periodic table.

### Step-by-Step Solution:

1. **Understanding Electron Gain Enthalpy**:
- Electron gain enthalpy is the amount of energy released when an electron is added to an atom in the gaseous state. A more negative value indicates a greater tendency to gain an electron.

2. **Trends in the Periodic Table**:
- As we move from left to right across a period, the electron gain enthalpy generally becomes more negative. This is due to increasing nuclear charge, which attracts the added electron more strongly.
- As we move down a group, the electron gain enthalpy becomes less negative. This is due to the increase in atomic size and the shielding effect, which makes it harder for the nucleus to attract the added electron.

3. **Analyzing O, S, and Se**:
- Oxygen (O) is in the second period, while sulfur (S) and selenium (Se) are in the third and fourth periods, respectively.
- Based on the trends:
- Oxygen will have a more negative electron gain enthalpy compared to sulfur and selenium because it is higher up in the group and has a smaller atomic radius, leading to less electron-electron repulsion when an electron is added.
- Sulfur will have a more negative electron gain enthalpy than selenium because it is above selenium in the group.

4. **Order of Electron Gain Enthalpy**:
- Therefore, the order of electron gain enthalpy (magnitude) from most negative to least negative is:
- O (most negative) > S > Se (least negative)

### Final Answer:
The order of electron gain enthalpy (magnitude) is: **O > S > Se**.