Final answer:
Iron is classified as a trace mineral because it is required by the body in quantities less than 100 milligrams per day and is essential for the health functions like forming hemoglobin.
Step-by-step explanation:
The Recommended Dietary Allowance (RDA) for iron is 8 mg for young men and 18 mg for young women. Iron is considered a trace mineral due to the fact that the body requires it in small amounts. Trace minerals, unlike major minerals, are needed by the body in quantities less than 100 milligrams per day. Iron is essential for various physiological processes, including the formation of hemoglobin, which is crucial for transporting oxygen in the blood. Due to its necessity in smaller quantities and vital roles it plays in health, it fits the classification of a trace mineral rather than a major mineral or any other designation like conditionally essential or possible essential.