Which of the following proteins is known as a hormone?

Insulin is classified as a hormone because it is produced by the pancreas and plays a critical role in regulating blood glucose levels. Hormones are signaling molecules that are secreted into the bloodstream and can affect target cells at a distance, influencing various physiological processes. Insulin specifically facilitates the uptake of glucose into cells, allowing them to use glucose for energy or store it as fat, thereby controlling blood sugar levels in the body.

Collagen, on the other hand, is a structural protein that provides strength and support to tissues and is not involved in hormonal signaling. Fibrin is a protein essential for blood clotting, playing a vital role in the process of hemostasis, but it is also not considered a hormone. Thrombin is an enzyme that converts fibrinogen into fibrin during the clotting process and is involved in blood coagulation rather than acting as a hormone. Thus, insulin stands out as the only hormone among the options provided.