The correct answer is option 2: hemoglobin. Hemoglobin is a protein found in red blood cells that carries oxygen throughout the body. It gives blood its red color because of the iron-containing heme groups within its structure. When oxygen binds to the heme groups, it forms oxyhemoglobin, resulting in the bright red color of oxygenated blood. When oxygen is released from hemoglobin, it becomes deoxygenated, resulting in darker red blood.

Option 1: Myoglobin is a protein found in muscle tissue that also carries and stores oxygen, but it is not responsible for the color of blood.

Option 3: Immunoglobulin, also known as antibodies, are proteins produced by the immune system to help fight infections. They are not responsible for the color of blood.

Option 4: Haptoglobin is a protein found in blood plasma that binds to and clears free hemoglobin released from damaged red blood cells. It does not contribute to the color of blood.