In general:

(i) First ionisation energy $\left({\mathrm{I}\mathrm{E}}_1\right)$ or enthalpy, increases along the period from left to right, but due to half-field configuration of P─atom, it has higher ${\mathrm{I}\mathrm{E}}_1$ than of S-atom.

(ii) ${\mathrm{I}\mathrm{E}}_1$ decreases as we move down the group from top to bottom.

(iii) More be the size of atom, less be the ${\mathrm{I}\mathrm{E}}_1$.

Hence, correct order of ${\mathrm{I}\mathrm{E}}_1$ is $\mathrm{B}<\mathrm{S}<\mathrm{P}<\mathrm{F}$.

${\mathrm{I}\mathrm{E}}_1$ for $\left(\mathrm{i}\mathrm{n}\mathrm{ }\mathrm{k}\mathrm{J}\mathrm{ }{\mathrm{m}\mathrm{o}\mathrm{l}}^{-1}\right)$

$\mathrm{B}=800$

$\mathrm{S}=999.4$

$\mathrm{P}=1012$

$\mathrm{F}=1680.8$