Which of the following is classified as an anticoagulant?

Heparin is classified as an anticoagulant because it directly inhibits blood coagulation by enhancing the activity of antithrombin III, a protein that inactivates certain enzymes involved in the clotting process, such as thrombin and factor Xa. By doing so, heparin reduces the blood's ability to clot, making it an effective treatment in the prevention and management of thromboembolic disorders, such as deep vein thrombosis and pulmonary embolism.

In contrast, the other options do not function as anticoagulants. Aspirin is an antiplatelet agent that works by inhibiting the aggregation of platelets, rather than preventing the formation of fibrin clots. Ibuprofen is a nonsteroidal anti-inflammatory drug (NSAID) that relieves pain and inflammation but does not have significant anticoagulant properties. Acetaminophen, while common for pain relief and fever reduction, also does not possess anticoagulant effects. Therefore, heparin stands out as the only medication among the options that is specifically used to inhibit clot formation.