\\bf{Hint:} A less stable compound or molecule will be more reactive. \\bf{Explanation:} The reactivity of any molecule or compound depends on its stability.A stable molecule does not react with other molecules until unless an external force like temperature, highly reactive reagent is not applied to it.In given molecules, ICl is the least stable molecule due to the large size difference as well as electronegativity difference between both atoms present in this molecule which makes it highly reactive.On other hand, Cl_{2} , Br_{2} and I_{2} are stable molecules at room temperature which makes these less reactive than ICl .Also in the case of Cl_2 , Br_2 and I_2 there is no electronegativity difference between the element so the molecule is non-polar so they are least reactive compared to ICl \\bf{Final \\ answer:} Option D