What protein involved in coagulation provides the activation for the final step in clotting?

The correct answer would be thrombin.

The last step in the blood clotting is the formation of a fibrin clot.

Thrombin is the serine protease (protein) enzyme which activates fibrin.

It converts the soluble fibrinogen into the insoluble fibrin which then results in the blood coagulation.

Blood is a bodily fluid in humans and other animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells. The cellular component of the blood responsible for blood clotting are the platelets. The blood clotting mechanism involves many substances like blood platelets, prothrombin, thromboplastin, Ca++, fibrinogen, vitamin K, etc.

Thromboplastin for blood clotting is secreted by platelets. In the presence of calcium, thromboplastin acts on protein prothrombin and forms a proteolytic enzyme called thrombin. Vitamin K is essential for formation of prothrombin in liver. Thrombin acts on a soluble protein fibrinogen and changes it into insoluble fibrin. Fibrin undergoes rapid polymerisation to form long fibres. The fibres form a network over the damaged wall of blood vessel and exposed part of the skin. It traps blood cells and forms a jelly like mass called blood clot. The clot contracts and solidifies. A liquid called serum is expelled.

Thus, the protein involved in coagulation provides the activation for the final step in clotting is the thrombin.