Which of the following is classified as an oxygen-carrying nutrient?

Folic acid is classified as an oxygen-carrying nutrient primarily because it plays a critical role in the formation of red blood cells. These cells are essential for transporting oxygen throughout the body. Folic acid, a B vitamin, is crucial for DNA synthesis and repair, which is fundamental in the development and maturation of red blood cells in the bone marrow. Inadequate folate levels can lead to megaloblastic anemia, a condition where the body produces abnormally large and ineffective red blood cells, resulting in decreased oxygen transport.

The other options, while beneficial to overall health, do not specifically function in the oxygen-carrying capacity of blood. Vitamin A is important for vision and immune function, Vitamin D is crucial for bone health and calcium metabolism, and Vitamin E serves as an antioxidant, helping to protect cells from damage. However, none of these vitamins have a direct role in the formation or function of red blood cells in the same way that folic acid does.