Question

what function do the kidneys serve within the excretory system?
detoxify harmful substances
remove carbon dioxide waste
remove oxygen waste
filter the blood to remove waste

Explanation:

The kidneys are a part of the excretory system. Their main function is to filter the blood. They remove waste products (like urea) and excess water from the blood, which is then excreted as urine. Detoxifying harmful substances is more of a liver function. Removing carbon dioxide is the job of the respiratory system (lungs). Oxygen is not a waste - it is essential for cellular respiration, and the body does not excrete oxygen as a waste product.

Answer:

Filter the blood to remove waste