Consider the elements $\mathrm{Mg}, \mathrm{Al}, \mathrm{S}, \mathrm{P} \mathrm{and} \mathrm{Si}$, the correct increasing order of their first ionisation enthalpy is :

In general, from left to right in a period, ionisation enthalpy increases due to an increase in effective nuclear charge.

But due to extra stability of half-filled (P) and full-filled electronic configuration $\left(\mathrm{Mg}\right)$, required ionisation enthalpy is more from neighbouring elements. i.e., first ionisation enthalpy order is

$\mathrm{Al}<\mathrm{Mg}<\mathrm{Si}<\mathrm{S}<\mathrm{P}$

Hence, the required answer is D.