The correct answer is option 'C', which states that the ascending limb of Henle's loop helps in the active reabsorption of sodium. Let's understand this in detail:

Nephrons are the functional units of the kidneys responsible for filtering blood and forming urine. Each nephron consists of several components, including the Bowman's capsule, proximal convoluted tubules (PCT), loop of Henle, and distal convoluted tubules (DCT).

- Bowman's capsule: It is the initial part of the nephron where blood is filtered. It contains a network of capillaries called the glomerulus, which filters small molecules such as water, ions, and waste products from the blood into the renal tubules.

- Proximal convoluted tubules (PCT): The filtered fluid (called filtrate) from the Bowman's capsule enters the PCT. The PCT is responsible for reabsorbing the majority of essential substances like glucose, amino acids, and water back into the bloodstream. However, sodium reabsorption in the PCT occurs passively, not actively.

- Loop of Henle: The loop of Henle is divided into a descending limb and an ascending limb. The descending limb is permeable to water but not to ions, while the ascending limb is impermeable to water but allows the reabsorption of ions, particularly sodium.

- Distal convoluted tubules (DCT): The DCT is responsible for fine-tuning the reabsorption and secretion processes. It plays a crucial role in regulating the body's electrolyte balance and pH. However, the active reabsorption of sodium primarily occurs in the ascending limb of Henle's loop.

The ascending limb of the loop of Henle actively reabsorbs sodium ions through a process called active transport. Active transport requires energy in the form of ATP and involves the movement of substances against their concentration gradient. In this case, sodium ions are actively transported out of the tubular fluid and into the surrounding interstitial fluid.

This active reabsorption of sodium ions creates a concentration gradient in the interstitial fluid, which allows for the passive reabsorption of other ions, such as chloride, as well as water in the descending limb of the loop of Henle.

In summary, the ascending limb of Henle's loop plays a crucial role in the active reabsorption of sodium ions, which is essential for maintaining the body's electrolyte balance and overall fluid homeostasis.