Final answer:
The element with the highest ionization energy in the given list is Potassium (option D).
Step-by-step explanation:
The ionization energy is the energy required to remove an electron from an atom. The element with the highest ionization energy in the given list will be one that is the most difficult to remove an electron from. In this case, the element with the highest ionization energy is Potassium, which is represented by option D.