Which of the following products is considered a source of incomplete proteins?

Incomplete proteins are those that do not contain all nine essential amino acids in sufficient amounts for the body's needs. Grains and nuts, while they are nutritious, are typically considered sources of incomplete proteins. Grains generally lack sufficient amounts of lysine, one of the essential amino acids, and nuts, while they can be high in protein, are also not complete sources on their own.

In contrast, other choices like meat and dairy, eggs and fish, and legumes with quinoa provide all of the essential amino acids or are complementary proteins when combined, thereby meeting the needs for a complete protein source. Therefore, grains and nuts are rightly recognized as sources of incomplete proteins.