Which of the following is classified as a trace element in food?

Iron is classified as a trace element in food because it is a necessary nutrient that the body requires in small amounts to function properly. Trace elements are essential for various physiological processes, including the formation of hemoglobin, which transports oxygen in the blood, and facilitating enzymatic reactions. The recommended daily intake of iron is relatively low compared to other nutrients, which aligns with its classification as a trace element.

In contrast, oxygen, carbon, and hydrogen are considered macronutrients or fundamental components of organic molecules. They are abundant in the body and are involved in essential biological processes, but they are not classified as trace elements due to their required quantities and roles in metabolic pathways.