Structural and functional unit of kidney
A. Neurons
B. Nephrons
C. Lungs
D. Schwans cells

Hint: Kidney is a bean shaped organ of the excretory system which leads to the formation of urine. These are responsible for removing wastes from the body and absorption of glucose and amino acids.

Complete answer:
-Kidney is the organ which is present in the excretory system. The organs help in the elimination of wastes from the body in the form of urine. Excess of salts like ammonium ions, potassium are excreted from the kidney. A Kidney consists of millions of nephrons which acts as the structural and functional unit. Each human kidney contains 1,000,000 nephrons.
-The function of nephrons is to produce urine during the removal of waste and excess toxic substances from the blood. This process facilitates the movement of water, molecules and ions from the walls of the capillaries. The ultra-filtrate forms do not have blood cells and broad molecules. This liquid passes through the tubes of the nephrons and gets collected in the urinary bladder and the process is known as ultrafiltration.

Neurons consists of Malpighian corpuscles and renal tubules:
-Malpighian or renal corpuscles: is made up of tufts of capillaries called glomerulus and a double layered cup shaped structure called Bowman’s capsules. Blood enters the glomerulus through an afferent arteriole and leaves it through an efferent arteriole.
-Renal tubules: is a long, convoluted tube that emerges from the glomerulus and it consists of Proximal Convoluted Tubule(PCT), Loop of Henle and Distal Convoluted Tubule( DCT). PCT is lined by cuboidal epithelial cells that bears a brush border of tail microvilli which increases the surface area. It helps in the reabsorption of salts by active transport. The loop of Henle consists of descending limb and ascending limb.
-The descending limb is permeable to water but impermeable to an electrolyte whereas the ascending limb is permeable to electrolytes but impermeable to water. DCT is the last part of the neurons and is lined by cuboidal cells. It collects and empties the content into the collecting duct. It also maintains sodium, potassium balance in the blood cells.

Hence, the correct answer is option (B).

Note: The neurons form a major part of the excretory system. They contain 35-55 mm long. The tubules of nephrons also perform absorption or reabsorption of water, glucose and other important components needed in the body. The rest of the waste matter that is not absorbed is excreted out of the body through the urine.