Final answer:
Iodine has the lowest ionization energy among the halogens listed, which follows the general periodic trend of decreasing ionization energy moving down a group.
Step-by-step explanation:
The halogen with the lowest ionization energy among the options given is iodine (d). Ionization energy generally decreases as you move down a group in the periodic table because the electrons are further from the nucleus and thus are less strongly attracted to it. Since iodine is lower in the halogen group compared to chlorine, bromine, and fluorine, it has the lowest ionization energy among the halogens listed.
As a point of reference among other available options:
- Potassium (K) belongs to Group 1.
- Helium (He) is a noble gas and has all its energy orbitals full.
- Calcium (Ca) has the smallest ionization energy when compared to options like potassium (K), phosphorus (P), and sulfur (S).