Name a common food source of vitamin B1 (thiamine).

Whole grains are a common food source of vitamin B1, also known as thiamine. This vitamin plays a crucial role in energy metabolism and the functioning of the nervous system. Whole grains such as brown rice, oats, and whole wheat bread provide a rich source of thiamine, as the vitamin is naturally present in the bran and germ of the grain.

Citrus fruits, while high in vitamin C and beneficial for overall health, do not have significant amounts of thiamine. Similarly, green leafy vegetables are more recognized for their content of vitamins A, C, and K, as well as minerals, but are not notable sources of vitamin B1. Dairy products primarily offer calcium, vitamin D, and some B vitamins, but thiamine is not one of their key nutrients. Thus, whole grains are the most reliable source of vitamin B1 among the options provided.