Generally, ionization enthalpy increases across a period from left to right due to increasing effective nuclear charge. Thus, we expect the order C < N < O < F. However, there is an anomaly between Nitrogen (Group 15) and Oxygen (Group 16). Nitrogen has a stable exactly half-filled electronic configuration ($1s^2 2s^2 2p^3$). Oxygen ($1s^2 2s^2 2p^4$) has two electrons paired in one 2p orbital, resulting in electron-electron repulsion that makes it easier to remove one electron compared to the stable configuration of Nitrogen. Therefore, the first ionization enthalpy of Oxygen is lower than that of Nitrogen. The correct order is C < O < N < F.