Final answer:
The ascending limb of the loop of Henle is impermeable to water, leading to a dilution of the filtrate as it ascends through active transport and passive diffusion of ions.
Step-by-step explanation:
The characteristic that best describes the ascending limb of the loop of Henle is that it is impermeable to water. Unlike the descending limb, which allows water to pass through, the ascending limb does not allow water to flow from the filtrate to the interstitial fluid. As a result, Na+ and Cl- ions exit through channels present in the cell membrane early in the ascending limb and further up, sodium is actively transported out of the filtrate, with chloride ions following. This process makes the filtrate more dilute as it ascends.
In summary, the ascending limb is responsible for removing ions from the filtrate without allowing water to follow, therefore its filtrate becomes more dilute, which is the correct answer to the question.