Looking for classes? Ksquare Career Institute, Bengaluru →
The first ionization enthalpy of O is lower than that of N and F.
Reason: The loss of an electron from O leads to stable half-filled p orbital
Detailed Solution
First ionisation enthalpy order: O < N < F, so the first IE of O is lower than both N and F. After removal of one electron, O becomes $2p^3$ (stable half-filled), which explains its lower IE. Both A and R are correct and R explains A.