Which mineral is necessary for oxygen transport in the blood?

The correct choice is iron, as it plays a crucial role in the transport of oxygen in the blood. Hemoglobin, the protein in red blood cells responsible for carrying oxygen from the lungs to the body's tissues, contains iron as an essential component. Each hemoglobin molecule can bind to oxygen due to the iron present in its structure. Without sufficient iron, the body cannot produce adequate amounts of hemoglobin, which can lead to reduced oxygen delivery to tissues and result in conditions such as anemia.

Calcium, while important for bone health and muscle function, does not participate directly in oxygen transport. Vitamin D is essential for calcium absorption and has roles in immune function, but it does not influence oxygen transport. Magnesium is involved in numerous biochemical reactions, including those related to muscle and nerve function, but it does not play a direct role in how oxygen is carried in the bloodstream. Thus, the necessity of iron for oxygen transport highlights why it is the correct answer.