Ionization energy is the minimum energy required to remove an electron from an isolated atom in the gaseous state, that is, in its ground state.
At the periodic table, we can see that there is a pattern for the first ionization energy.
It increases from bottom to top in a group, and from left to right in a period.
So:
Argon and Helium are on the far right of the periodic table, while sodium and potassium are on the far left.
So Argon and Helium have the highest ionization energy in comparison to Potassium and Sodium.
Between Argon and Helium, Helium is above argon in the periodic table, this means that helium has the highest ionization energy.
Answer: Helium