1. Meaning of electron gain enthalpy

Electron gain enthalpy is the enthalpy change when an isolated gaseous atom gains an electron: $X(g) + e^- \rightarrow X^-(g)$

For halogens, this process is generally exothermic, so electron gain enthalpy is negative.
The question asks for the absolute value, i.e. the magnitude.

2. General trend in halogens

As we move down the group from $F \to Cl \to Br \to I$:

• atomic size increases,
• incoming electron experiences less attraction from the nucleus,
• so the tendency to gain an electron generally decreases.

Thus one may expect: $F > Cl > Br > I$

But there is an important exception.

3. Why chlorine has greater electron gain enthalpy magnitude than fluorine

Although fluorine is smaller, its very small size means the added electron enters a compact $2p$ subshell where electron-electron repulsions are high.

In chlorine, the electron enters the larger $3p$ subshell, where repulsion is less. Therefore, adding an electron to chlorine releases slightly more energy than adding one to fluorine.

Hence, in terms of absolute value: $|\Delta_{eg}H|(Cl) > |\Delta_{eg}H|(F)$

4. Complete order

After chlorine, the magnitude decreases normally down the group: $Cl > F > Br > I$

5. Check options

• A: $Cl > Br > F > I$ ❌ because $F > Br$
• B: $Cl > F > Br > I$ ✅ correct
• C: $I > Br > Cl > F$ ❌ reverse trend
• D: $F > Cl > Br > I$ ❌ because $Cl > F$

Therefore, the correct option is: $\boxed{B}$