Which of the following is a waste product excreted by the kidneys?

The kidneys are one of the most important organs in the excretory system of the human body. They filter the blood and remove waste products, excess water, and salts from the body in the form of urine. One of the main waste products excreted by the kidneys is urea, which is a byproduct of the breakdown of proteins in the body. 

Carbon dioxide is a waste product excreted by the lungs, water is excreted by various organs including the kidneys, and oxygen is not a waste product but an essential component for respiration. 

Therefore, the correct answer is urea