Which of the following halogens is the least reactive?

Among the halogens, iodine is the least reactive element. The reactivity of halogens decreases as you move down the group in the periodic table, which is due to the increasing atomic size and the decreasing ability to gain an electron.

Fluorine, being the smallest halogen, is the most reactive because it has a strong tendency to gain an electron to achieve a full valence shell. Chlorine is less reactive than fluorine, and bromine has even less reactivity due to their larger atomic sizes and the increased distance between the nucleus and the valence electrons, which reduces the effective nuclear charge felt by incoming electrons.

As you proceed down the group from fluorine to iodine, the ability to attract an additional electron diminishes. Therefore, iodine's larger atomic structure makes it the least effective at accepting an electron, which firmly establishes its position as the least reactive halogen in comparison to fluorine, chlorine, and bromine.