The ionisation enthalpy increases as we go from left to right in a period, while it decreases as we come down a group. So according to this, the order will be Be<B<C<N<O. But there are two exceptions, in the case of Be and N because of the highly stable 2s2 (completely filled orbital) and 2s22p3 (half-filled orbital 2p3) valence configuration. Due to this, the correct order is:
B<Be<C<O<N.