What is the functional unit of the kidney?

The question asks for the functional unit of the kidney.

• A. A nephron is the microscopic structural and functional unit of the kidney, responsible for filtering blood and producing urine.
• B. A hepatocyte is a cell of the liver.
• C. A neuron is a nerve cell, the basic unit of the nervous system.
• D. An alveolus is a tiny air sac in the lungs where gas exchange takes place.

Based on these definitions, the correct answer is A.

The final answer is $\boxed{\text{A. Nephron}}$