**Core Concept**
The conversion of prothrombin to thrombin is a crucial step in the coagulation cascade, involving the interaction of several clotting factors with calcium ions. This process ultimately leads to the formation of a fibrin clot, which stabilizes the blood vessel and promotes hemostasis.

**Why the Correct Answer is Right**
The correct answer involves the participation of factor VIIa, which, in complex with tissue factor, activates factor IX and factor X. However, the direct conversion of prothrombin to thrombin involves the participation of factor Xa, along with calcium ions. Calcium ions facilitate the binding of factor Xa to the prothrombin complex, allowing for the conversion of prothrombin to thrombin. This process occurs in the presence of phospholipid surfaces, such as those provided by platelets or the endothelium.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify the correct factor involved in the conversion of prothrombin to thrombin.
**Option B:** This option is incorrect because it is not directly involved in the conversion of prothrombin to thrombin.
**Option C:** This option is incorrect because it is involved in the intrinsic pathway of coagulation, which is not directly related to the conversion of prothrombin to thrombin.

**Clinical Pearl / High-Yield Fact**
The conversion of prothrombin to thrombin is a vitamin K-dependent process, requiring the presence of vitamin K-dependent clotting factors (II, VII, IX, and X) for proper activation.

**Correct Answer:** C. Factor Xa.