Which statement about carotenoids in ornamental fish feed is true?

• A. They are proteins
• B. They are nitrates
• C. They are zooplankton
• D. They promote bright coloration

Answer: (D)

Carotenoids in ornamental fish feed are used to create and enhance the fish’s vivid colors. They’re pigments, not proteins, nitrates, or living organisms like zooplankton. Many ornamental fish can’t synthesize carotenoids on their own, so they rely on dietary sources. When these pigments are included in the diet, they deposit in the skin, fins, and scales, producing brighter reds, oranges, yellows, and other hues that are highly valued by hobbyists and sellers.

That’s why the statement about promoting bright coloration is the best choice: it directly describes the effect carotenoids have on appearance. The other options describe things that carotenoids are not—proteins are amino-acid polymers, nitrates are inorganic compounds, and zooplankton are living organisms—so they don’t fit.