(b) : As we move from left to right across a period, ionisation enthalpy increases with increasing atomic number. So the order of increasing ionisation enthalpy should be B < C < N < O.
But N(1s2 2s2 2p3 ) has a stable half filled electronic configuration. So, ionization enthalpy of nitrogen is greater than oxygen.
So, the correct order of increasing the first ionization enthalpy is B < C < O < N.