Final answer:
Chlorine (Cl) has the highest ionization energy among the given options.
Step-by-step explanation:
The ionization energy refers to the energy required to remove an electron from an atom or ion in the gaseous state. It represents the measure of the strength of the attraction between the negatively charged electrons and the positively charged nucleus of an atom or ion. The higher the ionization energy, the more difficult it is to remove an electron.
Among the given options, Chlorine (Cl) has the highest ionization energy. Chlorine is located in Group 17 of the periodic table, also known as the halogens, and it has a strong tendency to gain an electron to achieve a stable electron configuration. This means that it requires a significant amount of energy to remove an electron from chlorine.