The alkali metals are larger in size and have smaller nuclear charge thus they have lower ionisation enthalpy in comparison to alkaline earth metals. Therefore, option D is correct answer.

Lower ionization energies.

Ionization energy is the energy required to remove an electron from an atom or ion in the gaseous state. It is a measure of how tightly an electron is held by an atom or ion. The lower the ionization energy, the easier it is to remove an electron.

The alkali metals, which include elements such as lithium, sodium, and potassium, have lower ionization energies compared to the alkaline earth metals. This is due to the following reasons:

1. Atomic size:
The alkali metals have larger atomic radii compared to the alkaline earth metals. This is because alkali metals have one valence electron in their outermost energy level, whereas alkaline earth metals have two valence electrons. The larger atomic size of alkali metals results in weaker nuclear attraction, making it easier to remove an electron and lower the ionization energy.

2. Effective nuclear charge:
The effective nuclear charge is the net positive charge experienced by an electron in an atom. It is determined by the number of protons in the nucleus and the shielding effect of inner electrons. In alkali metals, the effective nuclear charge experienced by the valence electron is lower compared to alkaline earth metals. This is because the valence electron in alkali metals is shielded by inner electrons in a larger atomic size. The lower effective nuclear charge makes it easier to remove an electron, resulting in lower ionization energy.

3. Electron configuration:
The electron configuration of alkali metals ends in an s1 configuration, while alkaline earth metals end in an s2 configuration. The s1 configuration of alkali metals is more stable and has a lower energy, making it easier to remove the valence electron. On the other hand, the s2 configuration of alkaline earth metals is relatively more stable and requires more energy to remove the valence electrons.

Therefore, due to the larger atomic size, lower effective nuclear charge, and more stable electron configuration, the alkali metals exhibit lower ionization energies compared to the alkaline earth metals.