Which clotting factors are primarily affected by Heparin?

Heparin primarily affects the clotting factors involved in the intrinsic pathway of coagulation as well as thrombin. Specifically, it enhances the activity of antithrombin III, which leads to the inactivation of several clotting factors, including factor II (thrombin) and factor X.

In this context, option C is the correct answer since it accurately identifies factor II (thrombin) in conjunction with factors IX and XI, which are also influenced indirectly through the intrinsic pathway. The action of heparin on these factors leads to an anticoagulant effect, making it crucial in the management of thromboembolic disorders.

The other options do not accurately reflect the primary effects of heparin on the coagulation cascade. Factors such as 1 (fibrinogen), 2 (thrombin), 3 (tissue factor), 5, 6, 7, and 10 do not represent the direct impact of heparin. In summary, heparin's main influence is on factors that are more central to the intrinsic pathway and the thrombin generation process, which is why option C is the most accurate choice.