This nephron segment is not permeable to water even in the presence of ADH

The correct option is ascending limb of loop of Henle.

To understand why this is the correct answer, let us look at the water permeability characteristics of the different nephron segments and how they interact with Antidiuretic Hormone (ADH):
1. Descending limb of loop of Henle: This segment is highly permeable to water due to the continuous presence of aquaporin-1 water channels. Water moves out of the tubule passively into the hypertonic medullary interstitium. Its permeability to water is inherent and does not depend on ADH.
2. Ascending limb of loop of Henle: Both the thin and thick parts of the ascending limb are structurally and functionally impermeable to water because they completely lack water channels (aquaporins). Instead, this segment actively transport solutes (like Na⁺, K⁺, and Cl^-) out of the tubule. Since solutes are removed while water is trapped inside, the tubular fluid becomes progressively dilute. ADH has no effect on the water permeability of this segment, meaning it remains impermeable to water under all physiological conditions.
3. Collecting duct: The water permeability of the collecting duct is regulated and depends entirely on ADH. In the absence of ADH, it is impermeable to water. In the presence of ADH, aquaporin-2 water channels are inserted into the apical membranes of its cells, making it highly permeable to water and allowing water to be reabsorbed.

Therefore, the ascending limb of the loop of Henle is the segment that remains completely impermeable to water even in the presence of ADH.