The correct option is C Hairpin bend between descending and ascending limbs
Within the loop of Henle, urine is at the highest concentration at the hairpin bend between descending and ascending limbs. In a human kidney, the concentration of filtrate entering the loop is 300 milliosmols, and this concentration is multiplied to more than 1200 milliosmols at the bottom of the longest loops of Henle in the renal medulla.