The element most likely to react with chlorine (Cl) is Li (Lithium).
Lithium is an alkali metal and readily reacts with halogens like chlorine to form lithium chloride (LiCl). The other elements listed (Krypton, Oxygen, and Bromine) are less likely to react with chlorine. Krypton is a noble gas and is generally inert, while bromine is already a halogen and does not typically react with another halogen. Oxygen can form compounds with chlorine, but it does not have the same tendency to react as lithium does with halogens.