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 function and urine formation, sodium ions ($\text{Na}^+$) are reabsorbed via active transport mechanisms (like the sodium - potassium pump) to regulate osmolarity and fluid balance. Calcium and magnesium reabsorption in the kidneys is not as reliant on active transport for concentration in the same way sodium is.

Answer:

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