The correct option is A Nephrons
Nephrons are microscopic tube-like structures in the kidneys that filter the blood and cause wastes to be removed. They are functional units of the kidney. They are the most basic structures of the kidney's anatomy and are an integral part of the urinary system. Each kidney contains over one million of them. Nephrons are the main functional units of the kidneys. These tiny filtering units are responsible for removing harmful toxins and excess fluids from the blood, keeping it clean and maintaining its correct volume and concentration. Thus, option D is correct.