To solve this problem, we need to identify the factor responsible for the formation of concentrated urine. Let's analyze each option:$\newline \bullet$Option 1: Low levels of antidiuretic hormone (ADH) - ADH is responsible for increasing water reabsorption in the kidneys. Low levels of ADH would lead to dilute urine, not concentrated urine. Therefore, this option is incorrect.$\newline \bullet$Option 2: Maintaining hyperosmolarity towards inner medullary interstitium in the kidneys - The hyperosmolarity of the medullary interstitium is crucial for the reabsorption of water from the collecting ducts, leading to the formation of concentrated urine. This option is correct.$\newline \bullet$Option 3: Secretion of erythropoietin by the Juxtaglomerular complex - Erythropoietin is a hormone that stimulates red blood cell production and is not directly involved in urine concentration. Therefore, this option is incorrect.$\newline \bullet$Option 4: Hydrostatic pressure during glomerular filtration - While hydrostatic pressure is important for filtration, it does not directly influence the concentration of urine. Therefore, this option is incorrect.$\newline$Based on the analysis, the correct factor responsible for the formation of concentrated urine is:$\newline$Option 2: Maintaining hyperosmolarity towards inner medullary interstitium in the kidneys.