Each kidney has nearly one million complex tubular structures called ' Nephron ' which are functional units of kidney :)

Number of Nephrons in a Human Kidney:

Introduction:
The human kidney is a vital organ responsible for filtering blood and removing waste products from the body. It plays a crucial role in maintaining the body's overall balance and regulating various functions. Nephrons are the functional units of the kidney, responsible for the filtration and purification of blood. They are tiny structures that perform the essential task of removing waste products and excess water from the blood.

Nephrons in a Human Kidney:
- The correct answer is (D) millions nephrons.
- A human kidney contains millions of nephrons, which are responsible for the filtration and purification of blood.
- Each nephron consists of a renal corpuscle, proximal convoluted tubule, loop of Henle, and distal convoluted tubule.
- The renal corpuscle consists of a glomerulus and a Bowman's capsule, where the initial filtration of blood occurs.
- The proximal convoluted tubule reabsorbs essential substances such as glucose, salts, and water from the filtrate back into the blood.
- The loop of Henle creates a concentration gradient in the kidney, allowing for further reabsorption of water and salts.
- The distal convoluted tubule regulates the final concentration of urine by reabsorbing or secreting ions as required.

Reasoning:
- The human kidney needs to efficiently filter and process a large volume of blood to maintain proper bodily functions.
- Having millions of nephrons ensures that the filtration process is thorough and effective.
- The large number of nephrons allows for a higher surface area for filtration and increases the kidney's overall capacity to filter waste products and excess water.

Conclusion:
In conclusion, a human kidney contains millions of nephrons. These nephrons are responsible for the filtration and purification of blood, ensuring the removal of waste products and excess water from the body. The large number of nephrons in the kidney allows for efficient filtration and contributes to maintaining the body's overall balance.