The correct option is A Sodium
Secondary messengers are the low weight diffusible molecules that are used to relay signals within a cell. They are synthesised or released by specific enzymatic reactions, usually as a result of an external signal that is received by a transmembrane receptor. cAMP, cGMP, and Ca2+act as secondary messengers and are located within the cytoplasm.
Sodium helps to maintain blood volume and keeps nerves functioning.