The correct answer is B. Hemoglobin.
Hemoglobin is a protein found in red blood cells that carries oxygen from the lungs to the rest of the body. It is made up of four subunits, each of which contains an iron atom. When oxygen binds to the iron atom, it changes the shape of the hemoglobin molecule, which causes the blood to turn red.
Plasma is the liquid part of blood. It is made up of water, proteins, electrolytes, and other substances. Plasma does not contain hemoglobin, so it does not contribute to the red color of blood.
Red blood cells (RBCs) are the cells that carry oxygen in the blood. They are made up of hemoglobin, which is what gives blood its red color.
White blood cells (WBCs) are the cells that fight infection. They are not red, but they are a type of blood cell.
Therefore, the correct answer is B. Hemoglobin.