Remember:
"Facultative" always points to the Distal Convoluted Tubule (DCT), as it is regulated by hormones (aldosterone and ADH).
"Descending limb" is always permeable to water, never to electrolytes.
Step 1: List the four nephron regions and their signature jobs. The proximal convoluted tubule handles acid-base balance by secreting $H^+$ and ammonia. The descending limb of the loop of Henle lets water pass freely but not solutes. The distal convoluted tubule fine tunes sodium and water reabsorption under hormonal control. The collecting duct manages final $H^+$ and $K^+$ secretion. Step 2: Work function to region instead of region to function. Instead of starting from the segment names, start from each listed function and ask which single segment is famous for it, this avoids getting confused between similar sounding descriptions. Step 3: Pair them up. Acid-base secretion of ammonia points to the PCT, pure water permeability points to the descending limb, hormone dependent sodium-water handling points to the DCT, and the last ionic fine tuning points to the collecting duct. Final answer: Option 3, matching A-III, B-I, C-IV, D-II.
