Which organs are involved in elimination?

If you are talking about the human body, then you are referring to the Excretory Organ System. This system is involved in removing wastes that the body no longer needs. The kidneys are considered as the main excretory organs in humans because it helps the body eliminate excess water, urea, and other waste products in the form of urine. Other excretory organs include the skin, that releases sweat; the digestive system that removes insoluble materials through defecation; and the respiratory system that eliminates water vapor and carbon dioxide. 

A) early horses is your answer