Final answer:
Hemoglobin is a protein found in red blood cells that carries oxygen to cells and carbon dioxide to the lungs. It is composed of four symmetrical subunits and has iron associated with its heme groups.
Step-by-step explanation:
Hemoglobin is the correct answer to this question. Hemoglobin is a protein found in red blood cells that is responsible for carrying oxygen to cells and carbon dioxide to the lungs. It is composed of four symmetrical subunits and has iron associated with its heme groups. The iron in hemoglobin binds with oxygen, giving blood its red color. Some other options, such as myoglobin, chlorophyll, hemocyanin, and hemerythrin, are also involved in oxygen transport in different organisms but are not the main protein responsible in most vertebrates.