What chemical substance prevents clotting in a blood specimen?

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Preventing clotting in a blood specimen relies on an anticoagulant added to the sample. Heparin works by activating antithrombin III, which inhibits thrombin and factor Xa, stopping the conversion of fibrinogen to fibrin. This keeps the blood liquid for tests that require plasma. Hemolysin would damage cells rather than prevent clotting. Citric acid (citrate) also prevents coagulation by chelating calcium, but it acts differently from heparin. Sodium fluoride is mainly used to preserve glucose by inhibiting glycolysis and is not a primary anticoagulant. So, the substance that best fits the description is heparin.