This type of nephron is best to conserve water in body

The correct option is Juxtamedullary nephrons.

To understand why juxtamedullary nephrons are specialized for conserving water, we can look at the anatomical differences between the two main types of nephrons in the human kidney: cortical nephrons and juxtamedullary nephrons.
1. Cortical Nephrons: These make up about 85% of all nephrons. They have short loops of Henle that penetrate only weakly into the outer renal medulla. Consequently, they play a minor role in creating the concentration gradient in the medulla.
2. Juxtamedullary Nephrons: These constitute the remaining 15% of nephrons. Their glomeruli are located close to the corticomedullary junction, and they possess extremely long loops of Henle that extend deep into the renal medulla. They are also closely associated with specialized capillary loops called the vasa recta.

The long loops of Henle in juxtamedullary nephrons, along with the vasa recta, establish and maintain a high osmotic gradient (hyperosmolarity) in the interstitial fluid of the renal medulla through a mechanism known as the countercurrent multiplier system.
This high concentration gradient in the deep medulla allows for a large osmotic force to draw water out of the collecting ducts and back into the bloodstream under the influence of antidiuretic hormone (ADH). Therefore, juxtamedullary nephrons are essential for the production of concentrated urine and are the best type of nephron to conserve water in the body.