Which substance prevents clotting of the blood?

The selected answer, anticoagulant, is indeed the correct choice because anticoagulants are substances that inhibit or prevent the coagulation (clotting) of blood. They work by interfering with the blood-clotting process, thus ensuring that blood remains in a liquid state and preventing unnecessary blood clots from forming. This is crucial in medical settings, especially for patients who are at risk of developing thrombotic conditions, where clots can block blood vessels and lead to serious health issues like heart attacks or strokes.

In contrast, a catalyst generally refers to a substance that speeds up a chemical reaction without being consumed in the process; it does not play a role in blood clotting. Coagulants are agents that promote coagulation, which is the opposite of what is needed to prevent clotting. Hemoglobin is a protein in red blood cells responsible for carrying oxygen, and while it plays a vital role in respiratory physiology, it does not have a function related to the clotting or prevention of blood clotting. Thus, understanding the function of anticoagulants helps clarify why this option is vital in managing blood clotting disorders and maintaining blood flow.