Due to their electronic configuration... completely filled 's' orbital of alkaline earth metals. (ns2). and also outer most electron is very well screened from the nuclear charge of alkali metals

Ionisation potential of alkaline earth metal:
The ionization potential refers to the amount of energy required to remove an electron from an atom or ion in its ground state. In the case of alkaline earth metals, the ionization potential is higher compared to alkali metals.

Explanation:
1. Alkali metals:
- Alkali metals belong to Group 1 of the periodic table, which includes elements such as lithium, sodium, potassium, etc.
- These elements have a single valence electron in their outermost energy level, which is loosely held due to the low effective nuclear charge.
- As a result, it requires relatively less energy to remove this electron and form a cation (positively charged ion).
- Therefore, alkali metals have low ionization potentials.

2. Alkaline earth metals:
- Alkaline earth metals belong to Group 2 of the periodic table, which includes elements such as beryllium, magnesium, calcium, etc.
- These elements have two valence electrons in their outermost energy level, which are held more tightly compared to alkali metals.
- The effective nuclear charge experienced by these electrons is higher, making it more difficult to remove them and form cations.
- As a result, alkaline earth metals have higher ionization potentials compared to alkali metals.

Comparison:
- The ionization potential of alkaline earth metals is higher than that of alkali metals.
- The presence of an additional valence electron in alkaline earth metals increases the effective nuclear charge, making it more difficult to remove the electrons.
- The higher ionization potential of alkaline earth metals reflects the increased stability of their atoms due to the filled or half-filled valence electron configuration.

Conclusion:
The correct answer is option 'D', which states that the ionization potential of alkaline earth metals is more than alkali metals. This is due to the higher effective nuclear charge experienced by the valence electrons in alkaline earth metals, making it more difficult to remove them and form cations.