Final answer:
Legumes, grains, nuts, and other plant foods are sources of incomplete proteins, which can be combined to provide all essential amino acids.
Step-by-step explanation:
Foods that supply most but not all essential amino acids include legumes, grains, nuts, and certain other plant foods. These are considered incomplete proteins because they lack adequate amounts of one or more essential amino acids. To ensure a balanced intake of all essential amino acids, vegetarians and those on plant-based diets often eat a variety of these foods, which can complement each other. For example, rice and beans eaten together form a complete protein with all essential amino acids, despite each being incomplete on its own.