Step 1: Understand electron gain enthalpy trend.
Electron gain enthalpy generally becomes more negative across a period due to increasing nuclear charge, and becomes less negative down a group due to increasing atomic size and shielding effect. However, halogens show characteristic variations due to small size and electron-electron repulsions. Step 2: Identify relative values among halogens.
Among halogens:
- Chlorine has the most negative electron gain enthalpy due to optimal size and strong attraction.
- Fluorine is less negative than chlorine due to strong electron-electron repulsion in compact 2p orbitals.
- Bromine is less negative than chlorine but more negative than iodine.
- Iodine has the least negative value due to large atomic size and shielding. Step 3: Match given numerical values logically.
We match based on known trend:
- Chlorine → most negative → c (-349)
- Fluorine → slightly less negative → b (-328)
- Bromine → intermediate → d (-325)
- Iodine → least negative → a (-295) Step 4: Verify each pairing.
- I) Chlorine → c (-349) ✔
- II) Bromine → d (-325) ✔
- III) Fluorine → b (-328) ✔
- IV) Iodine → a (-295) ✔ Step 5: Final consistency check.
All values correctly follow periodic trends and known anomalous ordering among halogens due to electron-electron repulsion effects in fluorine. The mapping is consistent and physically valid. Final Answer:
\[
\boxed{(I)-c,\ (II)-d,\ (III)-b,\ (IV)-a}
\]
