Among the second-period elements $\mathrm{Li}$, $\mathrm{Be}$, $\mathrm{B}$ and $\mathrm{C}$, the lowest first ionisation enthalpy is shown by

The alkali metal has the largest atomic radius and lowest effective nuclear charge in the period, and its single $2s$ electron is lost to give a noble gas core. Values: Li 520, Be 899, B 801, C $1086\mathrm{kJ}\mathrm{mo}{\mathrm{l}}^{-1}$.