Generally, ionization enthalpy increases across a period from left to right due to increasing effective nuclear charge. However, there is an anomaly between Group 2 (Be) and Group 13 (B). Beryllium has a stable, completely filled valence shell configuration ($1s^2 2s^2$), and the 2s electrons are more penetrating towards the nucleus. Boron ($1s^2 2s^2 2p^1$) has a single 2p electron which is shielded by the inner 2s electrons, making it easier to remove. Therefore, the ionization enthalpy of Boron is lower than that of Beryllium ($B < Be$). The general trend holds for the rest, placing Lithium first (alkali metal) and Carbon and Nitrogen following the increase in nuclear charge. Nitrogen has higher stability due to a half-filled p-subshell, further increasing its value. The correct order is $Li < B < Be < C < N$.