Which of the following prevents the backflow of blood into the left atrium?

The mitral valve guards the atrioventricular aperture between the left atrium and left ventricle of the heart. It prevents blood from flowing back into the left atrium when the left ventricle contracts during ventricular systole. The mitral valve is made up of two flaps (hence the name bicuspid valve). 
The mitral valve is so named because it is shaped like a bishop's mitre, which is a type of pointed hat worn by bishops in the Catholic Church. The two flaps, or leaflets, of the mitral valve resemble the two peaks of the hat, and the central opening between the leaflets is shaped like the top of the hat.