This is responsible for production of concentrated urine

The correct option is juxtamedullary nephron.

Step-by-step Explanation:

1. Understanding Nephron Types:
The human kidney contains two major types of nephrons, classified by their anatomical location and structure:
- Cortical nephrons: These comprise about 85% of all nephrons. Their renal corpuscles lie in the outer portion of the renal cortex, and they have short loops of Henle that extend only shallowly into the outer renal medulla.
- Juxtamedullary nephrons: These make up the remaining 15% of nephrons. Their renal corpuscles lie deep in the cortex close to the medulla, and they feature very long loops of Henle that extend deep into the inner medulla.

2. Role of the Loop of Henle in Creating a Gradient:
The concentration of urine relies heavily on the osmotic gradient established within the interstitial fluid of the renal medulla. The long loops of Henle of the juxtamedullary nephrons, in conjunction with their associated capillary loops (the vasa recta), function as countercurrent multipliers and exchangers. This specialized system actively transports sodium and chloride ions out of the filtrate, establishing a hyperosmotic gradient that increases deep into the medulla.

3. Water Reabsorption and Urine Concentration:
As fluid flows through the collecting ducts towards the renal pelvis, it passes through this hyperosmotic medullary interstitium. In the presence of antidiuretic hormone (ADH), the collecting ducts become highly permeable to water. Water is osmotically drawn out of the collecting ducts and back into the surrounding capillaries, leaving behind a highly concentrated urine. Because cortical nephrons lack these long loops extending deep into the medulla, they cannot establish the steep osmotic gradient required for this process.

Thus, the juxtamedullary nephrons are structurally and functionally responsible for the kidney's ability to produce concentrated urine and conserve body water.