Final answer:
An example of complementary protein from the given options is 'pinto beans and vegetable casserole,' as it combines incomplete proteins to supply all essential amino acids when eaten together or throughout the day.
Step-by-step explanation:
Among the choices provided, an example of complementary proteins would be 'pinto beans and vegetable casserole,' as it combines legumes with grains or vegetables, which together provide all the essential amino acids. Complementary proteins are pairs of foods that contain incomplete proteins but together supply a full profile of essential amino acids. Examples of complementary protein pairings include pea soup and rye bread, beans and rice, and peanut butter on whole wheat bread. It is not necessary to eat complementary proteins in the same meal; consuming them throughout the day can still provide the benefits of complete protein intake.