During clot formation, thrombin A forms prothrombin B causes formation of fibrin threads C causes the release of clotting factors D forms a meshwork that clots the blood. - Scottish Highers Human Biology - Question 12 - 2017

Thrombin plays a critical role in the coagulation cascade. It is responsible for converting fibrinogen, a soluble plasma protein, into fibrin, which is an insoluble protein forming the meshwork that constitutes the bulk of a blood clot. This process is pivotal in stabilizing the clot and preventing excess blood loss. Therefore, option B correctly states that thrombin causes the formation of fibrin threads.