Which is an example of an anticoagulant medication?

The correct choice identifies an anticoagulant medication, which is primarily used to prevent blood clots by inhibiting the aggregation of platelets. Aspirin, in particular, is a non-steroidal anti-inflammatory drug (NSAID) that has antiplatelet properties. It works by blocking the enzyme cyclooxygenase, leading to a decrease in thromboxane A2 production, a promoter of platelet activation. This makes aspirin useful in various clinical settings, particularly for cardiovascular disease prevention.

Other options provided do not fit the classification of anticoagulants: Enalapril is an ACE inhibitor used primarily for managing hypertension and heart failure; Metformin is an oral hypoglycemic agent used in the management of type 2 diabetes; and Levothyroxine is a synthetic form of the thyroid hormone, used to treat hypothyroidism. Each of these medications serves different therapeutic purposes and mechanisms of action that do not involve anticoagulation.