Which of the following is an endothermic process?

Second electron affinity of any element is positive and endothermic. Because repulsion forces between the uni negative ion and electron.

${\mathrm{O}}_{\left(\mathrm{g}\right)}^{-}+{\mathrm{e}}^{-}\to {\mathrm{O}}_{\left(\mathrm{g}\right)}^{-2};{\mathrm{EA}}_2,∆\mathrm{H}=+\mathrm{ve};$ endothermic .process

Note: Formation of NaCl from gaseous ions is an exothermic

${\mathrm{Na}}_{\left(\mathrm{g}\right)}^{+}+{\mathrm{Cl}}_{\left(\mathrm{g}\right)}^{-}\to {\mathrm{NaCl}}_{\left(\mathrm{s}\right)}; \mathrm{Latticeenergy},\mathrm{U}= 787 {\mathrm{kJmol}}^{-1}$

Similarly, the hydration of ${\mathrm{MgCl}}_2$ is also an exothermic process.

${\mathrm{MgCl}}_{2\left(\mathrm{s}\right)}+2{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{l}\right)}\to \mathrm{Mg}{\left(\mathrm{OH}\right)}_{2\left(\mathrm{aq}\right)}+2{\mathrm{HCl}}_{\left(\mathrm{aq}\right)};∆\mathrm{H}=-\mathrm{ve}$

${\mathrm{Cl}}_{\left(\mathrm{g}\right)}+{\mathrm{e}}^{-}\to {\mathrm{Cl}}_{\left(\mathrm{g}\right)}^{-}; \mathrm{E}.{\mathrm{A}}_1,∆\mathrm{H}=-\mathrm{ve}: \mathrm{exothermic} \mathrm{process}$