This is used as an osmolyte in humans to increase medullary interstitial osmolality during concentration of urine

The correct option is Urea.

In the human kidney, the concentration of urine is a critical process governed by the medullary osmotic gradient. The renal medulla requires a high interstitial osmolality to draw water out of the collecting duct, thereby concentrating the urine. This gradient is maintained by two primary solutes: sodium chloride ($\mathrm{NaCl}$) and urea.

Urea acts as a major physiological osmolyte in humans. During periods of water conservation, antidiuretic hormone (ADH) increases the permeability of the inner medullary collecting ducts to urea via specific urea transporters (UT-A1 and UT-A3). This allows urea to passively diffuse out of the collecting duct and accumulate in the medullary interstitium, significantly raising its osmolality and enabling efficient water reabsorption.