Concept:The kidney is the main excretory and osmoregulatory organ in mammals.Explanation:The kidney filters the blood and regulates the amount of water, salts, hydrogen ions, and urea excreted in urine.It helps maintain homeostasis by balancing these substances in the body fluids.The bladder only stores urine, the colon absorbs water from digested food, and the liver processes urea but does not regulate its level in the blood.Answer:A. Kidney