The correct order of second ionization enthalpy of carbon, nitrogen, oxygen and fluorine is

After the removal of an electron from  C,N,O and  F, their electronic configurations are:C+1s22s22p1,N+1s22s22p2O+1s22s22px12py12pz1 and F+1s22s22px22py12pz1Since O+ gives a stable electronic configuration with exactly half – filled  2p-subshell, therefore,  IE2  of  O is higher than that of F. The  IE2  of  C because of the larger size is lower than that of N. Thus, the overall order is  O>F>N>C