Among $\mathrm{Li}$, $\mathrm{Be}$, $\mathrm{B}$ and $\mathrm{C}$, the element whose first electron gain enthalpy is positive is

Li ($-60$), B ($-27$) and C ($-122$ $\mathrm{kJ}\mathrm{mo}{\mathrm{l}}^{-1}$) release energy. Be has a filled $2s^2$ subshell, so the added electron must enter the higher energy $2p$ orbital, and the value is positive (about +48).