Which of the following is an example of a complete protein?

Quinoa is considered a complete protein because it contains all nine essential amino acids in sufficient quantities that the body cannot produce on its own. This makes it particularly valuable for vegetarians and vegans who may need to rely on plant-based sources for their protein intake. Unlike many plant proteins, which are often low in one or more essential amino acids, quinoa provides a balanced amino acid profile, making it an excellent choice for a nutritious diet.

In contrast, brown rice, peanut butter, and wheat bread do not contain all essential amino acids in adequate amounts on their own. For example, brown rice is generally low in lysine, and while it can pair well with other proteins to form a complete protein meal, it does not fulfill this requirement alone. Peanut butter, while a good source of protein, is also low in certain amino acids, making it an incomplete protein. Wheat bread shares similar limitations, as it lacks sufficient amounts of some essential amino acids as well. Therefore, quinoa stands out as the only option that meets the criteria of a complete protein.