The first ionization energies of alkaline earth metals are higher than those of the alkali metals. This is because 

Alkali metals belong to the IA group having electronic configuration ${\mathrm{ns}}^1$i.e, half-filled and unstable. Whereas, alkaline earth metals belong to IIA group having electronic configuration ${\mathrm{ns}}^2$which is fulfilled and so, is stable. 

The ns¹will readily try to lose its electron to attain the stable electronic configuration. And in alkaline earth metals (${\mathrm{ns}}^2$), the atomic radii decrease and the nuclear charge increases as it has a stable electronic configuration. Hence, the reason for higher first ionization energy of alkaline earth metals due to an increase in the nuclear charge.

Therefore, option (A) is correct.