d
Variation of electron gain enthalpy in periodic table-

1. Electron gain enthalpy decreases down the group.

2. generally, electron gains enthalpy increases (more negative) along the period.

Electron gain enthalpy increases along the period as the attraction between incoming electron and the nucleus increases.

Also, it decreases down the group due to increased atomic size.

\(\therefore\) Increasing order of electron gain enthalpy:

\(\mathrm{Ca} < \mathrm{Al} < \mathrm{C} < \mathrm{O} < \mathrm{F}\)