Fibrinogen to Fibrin Conversion:
The conversion of fibrinogen to fibrin is a crucial step in the process of blood clotting. It is catalyzed by an enzyme called thrombin.

Thrombin:
Thrombin is a serine protease enzyme that plays a central role in blood clotting. It is produced from its precursor molecule, prothrombin, which is activated by a series of enzymatic reactions known as the coagulation cascade. Thrombin is formed at the end of this cascade and is responsible for the final step in the clotting process.

The Role of Thrombin:
Thrombin acts on fibrinogen, a soluble plasma protein, and cleaves it into smaller units called fibrin monomers. These monomers then combine to form long, insoluble strands known as fibrin. Fibrin forms a mesh-like structure that traps blood cells and platelets, forming a stable blood clot.

Thrombin Activation:
Thrombin is activated by the enzyme complex known as prothrombinase. Prothrombinase is formed by the activation of factor X, which converts prothrombin to thrombin in the presence of calcium ions and phospholipids. This activation occurs on the surface of platelets and other cell membranes.

Thromboplastin:
Thromboplastin, also known as tissue factor, is a protein released from damaged tissues during injury or trauma. It plays a crucial role in initiating the coagulation cascade by activating factor VII, which then leads to the activation of factor X. Thromboplastin is not directly involved in the conversion of fibrinogen to fibrin.

Conclusion:
In conclusion, the conversion of fibrinogen to fibrin is catalyzed by thrombin, an enzyme produced from prothrombin. Thrombin is activated in a series of enzymatic reactions and acts on fibrinogen to form fibrin, which is the main component of blood clots. Thromboplastin, on the other hand, is involved in the initiation of the coagulation cascade but is not directly responsible for the conversion of fibrinogen to fibrin. Therefore, the correct answer is option 'A' - Thrombin.