The loop of Henle is a crucial part of the nephron, which is the functional unit of the kidney responsible for filtering blood and producing urine. It plays a vital role in the reabsorption of water and electrolytes, maintaining the osmotic balance of the body.

The loop of Henle is divided into three main segments: the thin descending limb, the thin ascending limb, and the thick ascending limb. It is responsible for creating an osmotic gradient in the medulla of the kidney, which is essential for the concentration and dilution of urine.

The complete loop of Henle is found only in mammals, making option 'D' the correct answer.

Here's a detailed explanation:

1. Functions of the Loop of Henle:
The loop of Henle performs two primary functions:
- Reabsorption of Water: The thin descending limb is permeable to water but not to solutes. As the filtrate moves down the descending limb, water is reabsorbed into the surrounding interstitial fluid, increasing its concentration.
- Reabsorption of Sodium and Chloride Ions: The thick ascending limb is impermeable to water but actively reabsorbs sodium and chloride ions into the interstitial fluid. This creates a high osmolarity in the medulla, crucial for the concentration of urine.

2. Structure and Segments:
The loop of Henle consists of three segments:
- Thin Descending Limb: This segment is lined with simple squamous epithelium and descends into the medulla. It is permeable to water but not to solutes.
- Thin Ascending Limb: This segment also consists of simple squamous epithelium but ascends back towards the cortex. It remains impermeable to water and solutes.
- Thick Ascending Limb: This segment is lined with cuboidal epithelium and actively transports sodium and chloride ions out of the filtrate into the interstitial fluid.

3. Osmotic Gradient and Countercurrent Exchange:
The loop of Henle is responsible for establishing an osmotic gradient in the medulla, which is crucial for the reabsorption of water and concentration of urine. It does so through a process called countercurrent exchange.
- As the filtrate moves down the descending limb, water is reabsorbed into the surrounding interstitial fluid, increasing its osmolarity.
- In the ascending limb, sodium and chloride ions are actively transported out of the filtrate, reducing its osmolarity.
- This creates a gradient of increasing osmolarity from the cortex to the medulla, allowing for the reabsorption of water in the collecting duct and the concentration of urine.

4. Significance in Mammals:
The complete loop of Henle is found only in mammals, including humans. It is longer and more elaborate in mammals compared to other vertebrates. This adaptation allows mammals to produce concentrated urine and conserve water more effectively, making them well-suited for various environments.

In conclusion, the complete loop of Henle is found in mammals, making option 'D' the correct answer. It is a crucial structure in the nephron responsible for the reabsorption of water and electrolytes, establishing an osmotic gradient in the medulla, and concentrating urine.