Most easily ${\mathrm{Fe}}^{2+}$ ion can be produced from $\mathrm{Fe}$ atom by absorbing energy from which for the following process . Consider equal no. given species .

The reaction is,

$\mathrm{Fe}⟶{\mathrm{Fe}}^{2+}+2e^{-}$

The quantity of energy released when an electron is added to a gaseous atom that is already present alone is known as the electron gain enthalpy. The size of chlorine is greater than that of Florine.

Thus, the required ion most easily can be formed with Formation of ${\mathrm{CI}}^{-}$from C- atom .