The correct answer is c) skin.
Worms, particularly earthworms, perform gas exchange primarily through their skin. Their skin is moist and serves as a surface for oxygen to enter the body and for carbon dioxide to be expelled. This process is known as cutaneous respiration. Unlike many other organisms, worms do not have specialized respiratory organs like gills or lungs, which makes their skin crucial for their ability to breathe. The moisture on their skin not only facilitates this gas exchange but also helps in the movement of the worms through soil and other substrates.