Which of the following elements has a positive first electron gain enthalpy?

Magnesium has the filled $3s^2$ subshell, so the added electron must enter the higher-energy $3p$ subshell and energy is absorbed (about $+39\mathrm{kJ}\mathrm{mo}{\mathrm{l}}^{-1}$). Cl, S and O all gain an electron exothermically (negative values).