Generally, the first ionisation energy increases along a period. But there are some exceptions. One which is not an exception is:

O has a larger nuclear charge, yet its ionisation enthalpy is lower than N's because N possesses a more stable, perfectly half-filled electronic structure while O does not. Similar to Mg, although having a larger nuclear charge, Al ionisation enthalpy is lower than that of Mg because Al requires the removal of a 3p- electron that is less strongly attracted. The 3s- electron must be eliminated. Similarly, although having a larger nuclear charge, B ionisation enthalpy is lower than Be because B requires the removal of a less strongly held 2p- electron, whereas Mg requires the removal of a more strongly held 2s- electron. But for Na and Mg, the ionisation enthalpy rises as the nuclear charge does.

 Hence, choice (b) is correct.