What is the primary role of sodium (Na) in the body?

Sodium plays a crucial role in the body primarily as an electrolyte, which helps maintain fluid balance and proper hydration in the cells and surrounding tissues. It is integral in regulating blood pressure and is involved in nerve transmission and muscle function. Sodium helps to balance the osmotic pressure across cell membranes, ensuring that fluids are properly distributed within and around cells.

While sodium is important for various physiological functions, its main function as an electrolyte highlights its vital role in maintaining homeostasis. Additionally, sodium contributes to the proper functioning of the nervous system by helping transmit electrical impulses, but this is more about its role in signaling rather than being a neurotransmitter itself.

Other options mention roles that sodium does not fulfill; for instance, sodium is not a catalyst or a vitamin, and it does not have a direct role in immune function. Thus, recognizing sodium's primary function in maintaining fluid balance clarifies why this is the correct choice.