**Core Concept**
Heparin is an anticoagulant medication that inhibits the coagulation cascade, preventing the formation of blood clots. It achieves this by activating a specific enzyme involved in the anticoagulant pathway.

**Why the Correct Answer is Right**
Heparin acts by activating Antithrombin III (AT III), a natural anticoagulant protein. When heparin binds to AT III, it enhances the enzyme's ability to inactivate thrombin and factor Xa, key enzymes in the coagulation cascade. This process effectively inhibits the coagulation pathway, preventing the formation of blood clots. The heparin-AT III complex is responsible for this anticoagulant effect.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because heparin does not activate Protein C, which is involved in the anticoagulant pathway but is not the primary mechanism of action for heparin.
* **Option B:** This option is incorrect because heparin does not activate Factor VII, which is a procoagulant enzyme involved in the initiation of the coagulation cascade.
* **Option C:** This option is incorrect because heparin does not activate Plasminogen, which is involved in fibrinolysis (the breakdown of blood clots) but is not the primary mechanism of action for heparin.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that heparin's anticoagulant effect is reversible with protamine sulfate, making it a crucial consideration in clinical settings where rapid reversal of anticoagulation is necessary.

**Correct Answer: C. Heparin acts by activation of Antithrombin III (AT III).**