JEE Main · 2021 · 26 Feb 2021 · Shift 1 · Classification of Elements and Periodicity

The four configurations belong to beryllium $(1s^{2}2s^2)$, oxygen $(1s^{2}2s^{2}2p^4)$, nitrogen $(1s^{2}2s^{2}2p^3)$ and boron $(1s^{2}2s^{2}2p^1)$.
Across the second period the ionisation enthalpy rises with nuclear charge, but two places break the rise. Boron falls below beryllium because its outermost electron sits in a $2p$ orbital that is screened by the filled $2s$ pair and is held less firmly than a $2s$ electron. Oxygen falls below nitrogen because oxygen loses one electron of its only paired $2p$ orbital and so relieves the repulsion between that pair, whereas nitrogen would have to break its stable half filled $2p^3$ set.
The order is therefore boron $<$ beryllium $<$ oxygen $<$ nitrogen, which assigns $801$ to $1s^{2}2s^{2}2p^1$, $899$ to $1s^{2}2s^2$, $1314$ to $1s^{2}2s^{2}2p^4$ and $1402\mathrm{kJ}\mathrm{mo}{\mathrm{l}}^{-1}$ to $1s^{2}2s^{2}2p^3$.