The main purpose of the chloride channel proteins on the apical surface of the intestinal epithelial cells is to create an osmotic gradient that ultimately causes to move through the intestinal epithelium and into the intestinal lumen to assist in the formation of mucus. These are the same chloride channel proteins, adversely affected in cystic fibrosis.

Water.

Explanation:

Cystic fibrosis may be defined as the medical condition in which the excess mucus is produced by the body that might affects the lungs and pancreas as well.

This occurs due to the abnormal functioning of chloride channel proteins. The main function of this protein is the transport of the water molecule that helps in the mucus formation. The water moves through the epithelial cells and maintain the role in epithelial fluid and fluid transport.

Thus, the answer is water.