Final answer:
Fluorine is the most reactive element among the given options because it is a member of the halogens group and has the highest electronegativity value and smallest atomic radius.
Step-by-step explanation:
The most reactive element among the given options (A. helium, B. bromine, C. chlorine, D. fluorine) is fluorine.
The elements of Group VIIA (new Group 17) are called the halogens and include fluorine, chlorine, bromine, iodine, and astatine. The halogens are highly reactive nonmetals. They have seven valence electrons, one electron short of having a full outer s and p sublevel, which makes them very reactive.
Fluorine is the most reactive of the halogens because it has the highest electronegativity value and the smallest atomic radius, which allows it to attract electrons more strongly and react more readily with other elements.