What is the the basic filtration unit in the human excretory system ?

Nephron 

The correct answer is Option (2) –Nephron

The excretory system of human beings includes a pair of kidneys, a pair of ureters, a urinary bladder and a urethra. Kidneys are located in the abdomen, one on either side of the backbone. Urine produced in the kidneys passes through the ureters into the urinary bladder where it is stored until it is released through the urethra .

The purpose of making urine is to filter out waste products from the blood. Just as CO₂ is removed from the blood in the lungs, nitrogenous waste such as urea or uric acid are removed from blood in the kidneys. It is then no surprise that the basic filtration unit in the kidneys like in the lungs, is a cluster of very thin-walled blood capillaries.. Each kidney has large numbers of these filtration units called nephrons packed close together. Some substances in the initial filtrate, such as glucose, amino acids, salts and a major amount of water, are selectively re-absorbed as the urine flows along the tube.In humans, a normal kidney contains 800,000 to 1.5 million nephrons. The nephron carries out all the functions of kidney and known as the functional unit of a kidney. It helps in filtration of blood from toxic materials.