The element most likely to react with chlorine (Cl) from the options provided is rubidium (Rb).
Rubidium is an alkali metal, which are known for being highly reactive, especially with halogens like chlorine. In contrast, sulfur (S) and silicon (Si) have lower reactivity, and bromine (Br) is also a halogen that is less likely to react with chlorine compared to rubidium. Thus, Rb is the best choice.