What compound, composed of iron and protein, is contained within red blood cells?

The compound composed of iron and protein that is contained within red blood cells is hemoglobin. Hemoglobin is a complex protein that plays a critical role in transporting oxygen from the lungs to the tissues and returning carbon dioxide from the tissues back to the lungs. Each hemoglobin molecule can bind to four oxygen molecules, thanks to the iron atoms contained in its heme groups, which allows for efficient oxygen delivery throughout the body.

In contrast, myoglobin, while also an iron-containing protein, is primarily found in muscle tissue and serves to store oxygen rather than transport it in the bloodstream. Fibrinogen is a soluble plasma protein important in the clotting process, and lactate is a byproduct of anaerobic metabolism, neither of which are involved in oxygen transport within red blood cells. Thus, hemoglobin is the specific protein found inside red blood cells that functions in oxygen transport, making it the correct answer.