Among the following, the process which requires maximum amount of energy is

$\mathrm{Na}\left(\mathrm{g}\right)\to {\mathrm{Na}}^{+}\left(\mathrm{g}\right)+{\mathrm{e}}^{-}$

The process which requires maximum amount of energy to remove an electron from the outer most shell  of an atom is called as ionization energy. Because ${\mathrm{Na}}^{+}$has stable electronic configuration removal of one electron from stable electronic configuration requires more energy.