$A$: The final reabsorption of water from urine into blood occurs through the collecting duct of a mammalian nephron,resulting in the production of hyperosmotic urine.
$R$: The loop of Henle is responsible for the formation of a sodium gradient across the depth of the medullary interstitium of a mammalian kidney.

• A
  Both Assertion and Reason are correct and Reason is the correct explanation of Assertion.
• B
  Both Assertion and Reason are correct but Reason is not the correct explanation of Assertion.
• C
  Assertion is correct,but Reason is incorrect.
• D
  Both Assertion and Reason are incorrect.