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Protamine sulfate is the specific antidote for heparin toxicity because it is a strong cationic protein that binds to heparin, neutralizing its anticoagulant effects. Heparin is an anticoagulant medication that works by enhancing the activity of antithrombin III, leading to the inhibition of thrombin and factor Xa, which in turn prevents blood clot formation. When too much heparin is administered or if there is an unexpected bleeding episode, protamine sulfate can be administered to reverse these effects by forming a stable complex with heparin and thereby rendering it inactive.

The other options do not serve as appropriate antidotes for heparin toxicity. Vitamin K is primarily used to reverse the effects of vitamin K antagonists such as warfarin, while activated charcoal is a treatment for certain types of drug overdoses and is not specific for heparin. Fresh frozen plasma can provide factors needed for clotting but does not directly counteract the effects of heparin. Thus, protamine sulfate is the most effective and appropriate choice for managing heparin toxicity.