The correct answer is a) fluorine. Halogens are a group of elements found in group 17 of the periodic table. They are known for their reactivity and include fluorine, chlorine, bromine, iodine, and astatine.
Fluorine is the first element in the halogen group and is highly reactive, making it the most electronegative element. The other options listed—oxygen, nitrogen, carbon, boron, beryllium, lithium, and iron—are not part of the halogen group and belong to different categories of elements.