Which of the following describes complete proteins?

Complete proteins are those that contain all nine essential amino acids that the body cannot synthesize on its own and must be obtained through diet. These proteins are particularly important for various functions such as tissue repair and muscle building. Meat sources, which include beef, poultry, pork, and fish, are traditional examples of complete proteins because they provide all these essential amino acids in adequate amounts.

While it's worth noting that some plant sources can also provide complete proteins (like quinoa and soy), the general characterization of complete proteins often highlights their presence in animal products. The other options either incorrectly define complete proteins or describe attributes that do not apply. For instance, plant sources can contain incomplete proteins (those lacking some essential amino acids), while saying they contain only carbohydrates misses the point of protein composition altogether.