Which of the following best describes red blood cells?

Red blood cells, also known as erythrocytes, are primarily responsible for transporting oxygen from the lungs to the tissues throughout the body. These cells contain a protein called hemoglobin, which binds to oxygen molecules, allowing for effective transport. As oxygen is delivered to cells, carbon dioxide, a waste product of metabolism, is picked up and carried back to the lungs for exhalation. This function is vital for maintaining cellular respiration and overall metabolic processes.

The roles of the other cell types mentioned serve important functions but do not accurately define red blood cells. For instance, cells that fight infection refer to white blood cells, which are part of the immune system. Similarly, the transportation of nutrients is primarily conducted by plasma and specialized cells like platelets play a crucial role in blood coagulation, forming clots to prevent excessive bleeding. Given this context, the primary function of red blood cells in oxygen transport distinguishes them clearly, making this option the best description among the choices provided.