Circulatory system isn't correct. Circulatory is the system that circulates blood. Therefore B is out.
C, respiratory, is not it. The respiratory system is responsible for inhaling oxygen and exhaling carbon dioxide.
The integumentary system is basically skin, in which protects our more sensitive body parts that allays underneath the skin from various potential harmful factors.
A should be the answer, because kidneys are like filters and separators. Separating needed things from waste and extra bodily fluid that would later be at disposal.
