**Core Concept**
Heparin is a low molecular weight anticoagulant that inhibits thrombin formation by activating antithrombin III (AT III), a natural anticoagulant protein. This process involves the heparin-AT III complex, which inactivates several coagulation enzymes, including thrombin (Factor IIa) and Factor Xa.

**Why the Correct Answer is Right**
The mechanism of heparin action involves the binding of heparin to AT III, which enhances the ability of AT III to inactivate thrombin and Factor Xa. This complexation is crucial for the anticoagulant effect of heparin. The heparin-AT III complex also activates protein C, which further contributes to the anticoagulant response. The activation of antithrombin III is the primary mechanism by which heparin exerts its anticoagulant effect.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because heparin does not directly activate protein C. While the heparin-AT III complex does activate protein C, this is an indirect effect.
**Option B:** This option is incorrect because heparin does not directly activate thrombin. In fact, heparin inactivates thrombin by binding to AT III.
**Option C:** This option is incorrect because heparin does not directly inhibit platelet aggregation. While heparin can affect platelet function, its primary anticoagulant effect is through the activation of AT III.

**Clinical Pearl / High-Yield Fact**
It is essential to note that unfractionated heparin (UFH) and low molecular weight heparin (LMWH) have different pharmacokinetic profiles and are used in different clinical settings. LMWHs have a more predictable anticoagulant response and are often used in outpatient settings.

**Correct Answer: B. activating antithrombin III**