______ is the structural and functional unit of kidneys. 

Nephron

The correct answer is Option (3) – Nephron

The nephron is the structural and functional unit of the kidneys. It is responsible for filtering the blood, reabsorbing essential substances, and secreting waste products to form urine. Each kidney contains millions of nephrons, which play a crucial role in maintaining the body's internal environment by regulating water and electrolyte balance, pH, and blood pressure.