An example of an anticoagulant rodenticide is:

Warfarin is an example of an anticoagulant rodenticide because it works by inhibiting the synthesis of vitamin K-dependent clotting factors in the liver, ultimately leading to internal bleeding in rodents. This mechanism is effective for controlling rodent populations, as the affected rodents usually die from blood loss after consuming a lethal dose of the substance.

Other substances listed, such as red squill and cyanide gas, operate through different toxicological mechanisms and are not classified as anticoagulants. Red squill is a plant-based rodenticide that acts as a stomach poison, while cyanide gas is a fast-acting lethal chemical that disrupts cellular respiration, leading to rapid death. Malathion is an organophosphate pesticide that affects the nervous system of insects, not a rodenticide, thereby distinguishing it further from anticoagulants.

The specific action of warfarin on blood coagulation is what categorizes it as an anticoagulant rodenticide, making it the correct answer in the context of this question.