Sodium plays a major role in:

Sodium plays a crucial role in fluid balance, which is essential for maintaining homeostasis in the body. It is a key electrolyte that helps regulate the amount of water in and around cells. Sodium is involved in the osmotic gradient that drives fluid movement, ensuring that tissues receive adequate hydration while preventing excess fluid accumulation. This regulation is vital in processes such as maintaining blood pressure and ensuring efficient kidney function, where sodium helps control the excretion and reabsorption of water.

While other choices mention important bodily functions—energy production, bone health, and muscle repair—these processes are not directly related to the primary role of sodium. Energy production involves primarily carbohydrates and fats, bone health is greatly influenced by calcium and phosphorus, and muscle repair focuses on proteins and amino acids. Therefore, the significance of sodium in maintaining fluid balance stands out as its primary function, making it the correct answer.