Which electrolyte works with sodium to maintain fluid balance and forms hydrochloric acid in the stomach?

Chloride is the correct answer because it plays a crucial role in maintaining fluid balance in the body alongside sodium. These two electrolytes work in tandem to regulate osmotic pressure and ensure that fluids shift between various compartments in the body appropriately. Additionally, chloride is a component of hydrochloric acid (HCl), which is produced in the stomach and is essential for digestion. Hydrochloric acid aids in breaking down food, activating digestive enzymes, and killing harmful bacteria that might enter the gastrointestinal tract. This dual function of chloride in fluid balance and digestion underscores its importance in the body.

The other options, while important in various physiological functions, do not directly relate to both the maintenance of fluid balance in conjunction with sodium and the formation of hydrochloric acid.