The correct answer is option 2: Kidneys. In mammals, the kidneys play a vital role in excretion. They are responsible for filtering waste products from the blood, such as urea, uric acid, and ammonia, and converting them into urine. The kidneys also help in regulating the body`s fluid and electrolyte balance.

Option 1: Large intestine is not involved in excretion. Its main function is to absorb water and electrolytes from undigested food and to eliminate solid waste in the form of feces.

Option 3: Lungs are primarily responsible for the exchange of gases, such as oxygen and carbon dioxide, during respiration. However, they do not play a significant role in excretion.

Option 4: The liver performs various functions, including detoxifying harmful substances and producing bile for digestion. While it does help in eliminating some waste products from the blood, its main role is not in excretion.

Therefore, the kidneys are the organs that play the most important role in excretion in mammals.