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 major part of the excretory system. Their main function is to filter the blood, removing waste products and excess substances to form urine. Detoxifying harmful substances is more of a liver - related function. Removal of carbon dioxide is mainly the role of the respiratory system. Oxygen is not a waste product in the context of the excretory system.

Answer:

Filter the blood to remove waste