Final answer:
The filtrate becomes more concentrated in the loop of Henle, with the descending limb absorbing water and the ascending limb removing solutes, particularly Na+ and Cl- ions. This countercurrent multiplier system aids in the concentration of the filtrate and helps maintain water and electrolyte balance.
Step-by-step explanation:
The filtrate becomes more concentrated as it traverses the loop of Henle in the kidneys. In the descending limb, which is permeable to water but not to solutes, water is absorbed by the high salt concentration of the renal medulla, increasing the filtrate's concentration. As the filtrate enters the ascending limb, which is permeable to solutes but not to water, Na+ and Cl- ions are removed. The active transport of these ions out of the filtrate contributes to an increasing osmotic gradient within the medulla, further concentrating the filtrate. If the active transport of NaCl stopped, the osmolarity of the filtrate in the ascending limb would decrease (option c). This countercurrent multiplier system ensures efficient reabsorption of water and solutes, crucial for maintaining water and electrolyte balance within the body.