Question

many aquatic birds secrete waxy organic substances that repel water. the birds use these substances to coat their feathers. an analysis of these substances would reveal that they are mostly composed of...
lipids
proteins
carbohydrates
nucleic acids

Explanation:

Lipids are hydrophobic (water - repelling) substances. Aquatic birds secrete waxy organic substances to coat their feathers to repel water, and these substances are mainly lipids. Proteins, carbohydrates, and nucleic acids do not have the same water - repelling properties as lipids in this context.

Answer:

lipids