What substance is formed when thrombin converts fibrinogen?

When thrombin converts fibrinogen, the substance that is formed is fibrin. This process is an essential step in the clotting cascade. Thrombin acts on fibrinogen, a soluble plasma protein, and cleaves it into fibrin monomers. These monomers then polymerize to form a stable fibrin mesh, which provides a scaffold for platelets to adhere to and ultimately leads to the formation of a stable blood clot.

Fibrin is crucial in hemostasis as it strengthens the temporary platelet plug that forms during initial blood vessel injury. The formation of fibrin is what allows for the effective sealing of the damaged area and prevents excessive blood loss.

The other options are not formed directly through the conversion of fibrinogen by thrombin, hence they do not represent the correct answer.