The correct option is C Mg
Ionisation energies of IIA group elements are greater than IA group
(Mg,Ca)>(Na,K)
Also,among IIA group elements first ionisation energy gradually decreases as we descend the group.
Mg>Ca>Na>K
Since energy is inversely proportional to wavelength, a radiation of the shortest wavelength means that energy should be high. The one with the highest ionisation energy among the given elements is Mg.