Which of the following minerals is known to be essential for fluid balance in the body?

Sodium is known to be essential for fluid balance in the body because it plays a critical role in maintaining osmotic pressure and fluid distribution within and outside of cells. It is a key electrolyte that helps regulate blood volume, blood pressure, and overall fluid balance. Sodium works together with other electrolytes, such as potassium and chloride, to create an appropriate environment for nerve impulses, muscle contractions, and hydration. When sodium levels are adequate, they help ensure that the body maintains proper hydration and nutrient transport.

In contrast, magnesium, iron, and calcium serve other primary functions in the body. Magnesium is important for energy production and muscle function; iron is crucial for oxygen transport in the blood; and calcium is primarily associated with bone health and muscle contraction. While these minerals have important roles, they do not have the same direct effect on fluid balance as sodium does.