**Core Concept**
The question is testing the student's knowledge of anticoagulant mechanisms, specifically the role of calcium in blood coagulation and the use of chelating agents to prevent clotting.

**Why the Correct Answer is Right**
The correct answer is EDTA (ethylenediaminetetraacetic acid), a chelating agent that binds to calcium ions, thereby inhibiting the coagulation cascade. Calcium is essential for the activation of several enzymes in the coagulation pathway, including prothrombinase and factor Xa. By chelating calcium, EDTA prevents the formation of a stable fibrin clot and is used as an anticoagulant in laboratory settings, particularly for blood banking and coagulation testing.

**Why Each Wrong Option is Incorrect**
* **Option A:** Heparin is an anticoagulant that works by activating antithrombin, which inhibits thrombin and factor Xa. It does not chelate calcium.
* **Option B:** Warfarin is a vitamin K antagonist that inhibits the synthesis of clotting factors in the liver. It does not directly chelate calcium.
* **Option C:** Aspirin inhibits platelet aggregation by irreversibly inhibiting cyclooxygenase, which reduces thromboxane A2 production. It does not chelate calcium.

**Clinical Pearl / High-Yield Fact**
Remember that EDTA is a potent chelating agent and should be used with caution, as it can also interfere with other laboratory tests, such as those measuring ionized calcium levels.

**Correct Answer:** D. EDTA