Correct Answer: Helium

Solution : The correct option is Helium.

Ionisation energy is the energy required to remove an electron from a neutral atom, forming a positively charged ion. Helium has the highest ionisation energy because it has the smallest atomic size among all the elements and a full electron shell (1s²), making it very stable and resistant to losing its electrons. This results in a high energy requirement to remove an electron from a helium atom.