Which description best defines a complete protein source?

• A. Plant sources that include all nine essential amino acids
• B. Soy is the only protein that contains all essential amino acids
• C. Comes from animals and has all nine essential amino acids
• D. Contains all essential amino acids but is derived from only grains

Answer: (C)

Complete proteins provide all nine essential amino acids in adequate amounts. Animal-derived foods—such as meat, fish, eggs, and dairy—consistently deliver all nine amino acids in the right balance, making them reliable complete protein sources. Many plant foods do not offer all nine essential amino acids by themselves, though a few do (like soy or quinoa); in contrast, the general statement that plant sources inherently contain all nine is not accurate. So describing a complete protein as coming from animals with all nine essential amino acids best matches the standard definition.