Which valve prevents the backflow of blood into the left ventricle when the ventricles relax?

Answer: Semilunar Valves

Explanation: Semilunar Valves

The semilunar valves allow blood to be pumped into the major arteries while preventing backflow of blood from the arteries into the ventricles.

The semilunar valves are located at the connections between the pulmonary artery and the right ventricle, and the aorta and the left ventricle. These valves allow blood to be pumped forward into the arteries, but prevent backflow of blood from the arteries into the ventricles. These valves do not have subvalvular apparatus and are more similar to the semilunar valves in veins and lymphatic vessels than to atrioventricular.

The semilunar valves allow blood to be pumped into the major arteries while preventing backflow of blood from the arteries into the ventricles.