Final answer:
The halogen with the highest ionization energy is F (Fluorine) due to its small atomic radius and high effective nuclear charge.
Step-by-step explanation:
The halogen with the highest ionization energy is F (Fluorine). This is because ionization energy increases across a period from left to right on the periodic table. Fluorine, being the first halogen on the periodic table, has the highest ionization energy compared to other halogens like chlorine, bromine, and iodine. Its small atomic radius and high effective nuclear charge make it difficult to remove an electron from a fluorine atom, resulting in a high ionization energy.