Which of the following factors is responsible for the formation of concentrated urine?

Correct option is C. Maintaining hyperosmolarity towards inner medullary interstitium in the kidneys. The proximity between loop of Henle and vasa recta as well as counter current in them help in maintaining an increasing osmolarity towards the inner medullary interstitium. This mechanism helps to maintain a concentration gradient in medullary interstitium so human urine is nearly four times concentrated than initial filtrate formed. So, the correct answer is 'Maintaining hyperosmolarity towards inner medullary interstitium in the kidneys'.