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Hemoglobin is a protein – not an enzyme. This is a small difference, but it is important because it emphasizes the idea that not all proteins need to be enzymes. In this case, hemoglobin is not an enzyme because it does not catalyze a specific reaction; instead, it just transports oxygen around the body to the places that need it (think about the systemic and pulmonary circuits). 

From there, we are told that hemoglobin has four subunits; this is referring to the fact that the hemoglobin is made of interactions between four different polypeptides. This relates directly to quaternary structure of the protein. 

And lastly, heme is a prosthetic group, which is something that you ought to memorize simply because it is something that is prevalent and bound to show up on both midterms and standardized tests. 

All of the information above matches answer choice A.