Lowest ionisation potential will be of -

CONCEPT:

Ionization Potential

• Ionization potential (also known as ionization energy) is the energy required to remove an electron from a neutral atom in its gaseous state.
• The lower the ionization potential, the easier it is to remove an electron from the atom.
• Factors affecting ionization potential include atomic size, nuclear charge, and the electron configuration of the atom.

EXPLANATION:

• Alkali metals have the lowest ionization potential because they have a single electron in their outermost shell, which is relatively easy to remove due to the large atomic size and low nuclear charge felt by the valence electron.
• Inert gases have very high ionization potential because their outer electron shell is complete, making them very stable and resistant to losing an electron.
• Halogens and d-block elements have higher ionization potentials compared to alkali metals because their electrons are more tightly bound due to higher nuclear charge and smaller atomic size.

CONCLUSION:

The correct answer is Alkali metals.