Answer:
E. integumentary
Explanation:
The integumentary system is consisted of hair, feathers, skin, glands, nerves, nails, with its main function being to act as a protective barrier of the body from the outside world. It helps in regulation of the body temperature, retaining body fluids, elimination of waste products, and protection form diseases. The waterproof feathers of the ducks are a nice example of this. They manage to keep their bodies warm in the water, thus helps them a lot in the regulation of the body temperature by being a barrier between the body and the water.
