The descending limb of Henle's loop plays a crucial role in the formation of concentrated urine by selectively reabsorbing water from the filtrate. This segment of the nephron is permeable to water but impermeable to electrolytes, which allows for the establishment of an osmotic gradient in the medulla of the kidney.

Below is a detailed explanation of why option 'C' is the correct answer.

Function of the Descending Limb
The descending limb of Henle's loop is responsible for the reabsorption of water from the filtrate. As the filtrate flows down the descending limb, it encounters an increasing osmolarity in the interstitial fluid of the medulla. This osmotic gradient is generated by the active transport of sodium and chloride ions out of the ascending limb and into the interstitium.

Permeability to Water
The descending limb is highly permeable to water due to the presence of aquaporin channels in the cell membrane of its epithelial cells. These channels allow water to passively move out of the tubule and into the interstitium, following the osmotic gradient. As a result, water is reabsorbed from the filtrate, which becomes more concentrated as it descends deeper into the medulla.

Impermeability to Electrolytes
Unlike the ascending limb, the descending limb is impermeable to electrolytes such as sodium and chloride ions. This impermeability is due to the absence of specific transporters or channels that would facilitate the movement of these ions across the cell membrane. As a result, the concentration of electrolytes in the filtrate remains relatively constant as it travels down the descending limb.

Establishment of Osmotic Gradient
The selective reabsorption of water from the descending limb creates a concentration gradient in the interstitium of the medulla. This gradient, with higher osmolarity at the bottom of the loop, is essential for the subsequent reabsorption of electrolytes in the ascending limb. The high concentration of solutes in the interstitium allows for the passive reabsorption of sodium and chloride ions in the ascending limb, leading to further concentration of the filtrate.

In conclusion, the descending limb of Henle's loop is permeable to water but impermeable to electrolytes. This permeability pattern enables the reabsorption of water from the filtrate and the establishment of an osmotic gradient, contributing to the formation of concentrated urine.

Loop of Henle: In the descending limb (impermeable to electrolytes), as the filtrate moves, water comes out, and the filtrate becomes concentrated (hypertonic) while in ascending limb (impermeable to water) as the filtrate moves, electrolytes come out, and the filtrate becomes dilute (hypotonic).