Explanation

(b)
Om moving along a period, ionisation enthalpy increases. Thus, the order of ionisation enthalpy should be as follow :
$F > O > N$
But N has half-filled structure, therefore, it is more stable than O, That’s why its ionisation erthalpy is highper than O. Thus, the correct order of IE is
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ F > O > N$