The halogens are elements found in Group 17 of the periodic table and include fluorine, chlorine, bromine, iodine, etc. Their reactivity is related to their electron affinity or the energy change associated with the gaining of an electron by an atom.

Option 1: Fluorine is the most reactive halogen because it has the highest electron affinity among the halogens. It readily accepts additional electrons to achieve the stable electronic configuration, making it highly reactive.

Option 2: Chlorine, while still quite reactive, is less reactive than fluorine. This lower reactivity is due to its larger atomic size compared to fluorine, leading to lower effective nuclear charge on the outermost electrons.

Option 3: Bromine is even less reactive than chlorine. Its larger atomic size compared to chlorine weakens the effective nuclear charge on outermost electrons.

Option 4: Iodine is the least reactive of all the options given. This is due to its large atomic size, which significantly weakens the effective nuclear charge, making it least interested in accepting electrons.

In summary, as atomic size increases down the group in the periodic table (from Fluorine to Iodine), reactivity decreases. Hence, the correct answer is fluorine.