For elements  $B,C,N,Li,Be,O$ and $F,$  the correct order of first ionisation enthalpy is

The correct order of first Ionisation Energy is
 $Li<B<Be<C<O<N<F$
On moving from left to right in a period atomic size decrease, eff increases, so ionization enthalpy increases but pesence of half filled and fully filled configuration will make on ion more stable thus more will be ionization enthalpy thus the order is  $Li<Be>C<N>O<F$