The nephron that is most responsible for urine concentration is b) juxtamedullary nephrons.
Juxtamedullary nephrons are unique as they have longer loops of Henle that extend deep into the medulla of the kidney. This anatomical feature enables them to create a more concentrated urine through the process of countercurrent multiplication. As the filtrate descends and ascends through the loop of Henle, water and sodium are reabsorbed selectively. This gradient is crucial for concentrating urine and conserving water, particularly during periods of dehydration. In contrast, cortical nephrons have shorter loops and play a lesser role in this concentration process.