The kidneys are bean-shaped organs located in the abdominal cavity. They are concerned with excretion, passing on the toxic and unwanted products to urine, to be sent out of the body.
Ureters (A) are the ducts that lead from the kidney to the bladder.
Glomeruli (B) are a constituent of the renal corpuscle.
Renal capsule (D) is a fibrous covering of kidney.
The above are parts of the kidney but they are not the functional unit of kidney.
Therefore, options A, B and D are incorrect.
The Nephron (C) is the structural and functional unit of kidney. It functions to regulate the concentration of water and soluble substances like sodium salts by filtering the blood and reabsorbing what is needed and excreting the rest as urine. There are about a million nephrons in each kidney. Each nephron is a long tube (~50 mm long) with a double walled cup like structure called Bowman’s capsule at one end. It encloses a cluster of blood vessels/capillaries called the glomerulus. Glomerulus and Capsule together constitute the renal corpuscle.
Therefore, option (C) is correct.