Option C is correct because the textbook explicitly states that 'The excretory system of human beings includes a pair of kidneys, a pair of ureters, a urinary bladder and a urethra.' The kidneys function to filter out waste products from the blood, specifically removing nitrogenous waste such as urea or uric acid. This process is known as excretion. Options A (nutrition), B (respiration), and D (transportation) are incorrect because the kidneys are not involved in these processes—nutrition relates to the digestive system, respiration involves the lungs and respiratory tract, and transportation refers to the circulatory system.