In the context of hemostasis, the von Willebrand factor (VWF) plays a crucial role in the formation of a platelet plug at sites of vascular injury. When endothelial cells are damaged, collagen fibers are exposed, and VWF acts as a bridge, facilitating the adhesion of platelets to these exposed collagen sites. This interaction is essential for initiating the process of platelet plug formation, which is a critical step in stopping bleeding.

While other substances such as adenosine diphosphate (ADP), serotonin, and thromboxane A2 are important in the later stages of platelet activation and aggregation, they do not serve as the initial adhesive factor. Instead, they are released during the activation and degranulation of platelets after they have adhered to the damaged site. Therefore, in response to the question regarding which molecule acts as the glue between exposed collagen and damaged endothelial cells and platelets, the correct answer is the von Willebrand factor.