Answer:
S (sulfur)
Explanation:
In the Periodic Table, the ionization energy increases from bottom to top in a group and from left to right in a period.
The chemical elements Na (sodium), Mg (magnesium), Al (aluminium), and S (sulfur) are all in the same period. Sulfur (S) is the element located more at the right in the period. Thus, S has the highest ionization energy.
