Question

which ion that is part of urine formation relies on active transport to concentrate in the kidneys? calcium (ca⁺²) sodium (na⁺) magnesium (mg⁺²)

Explanation:

In kidney physiology, sodium ions ($\text{Na}^+$) are reabsorbed in the kidneys (part of urine formation) via active transport mechanisms, especially in the renal tubules, to maintain electrolyte balance and concentrate in the kidneys. Calcium and magnesium reabsorption have different mechanisms and are not as reliant on active transport for concentration in the kidneys as sodium is.

Answer:

B. sodium ($\text{Na}^+$)