Which source provides a complete protein?

Quinoa is recognized as a complete protein because it contains all nine essential amino acids that the human body cannot synthesize on its own. Essential amino acids must be obtained through dietary sources, and quinoa provides these in adequate amounts, making it an excellent choice for vegetarians and those seeking plant-based protein options.

While peanut butter, rice, and wheat are nutritious in their own rights, they do not provide the full spectrum of amino acids needed to be classified as complete proteins. Peanut butter lacks lysine, rice is low in lysine and several other essential amino acids, and wheat is limited in threonine and lysine. Therefore, quinoa stands out as the only option that fulfills the requirement of being a complete protein source.