What valve prevents back flow into the left ventricle of the heart? a) pulmonary

Option D, aortic valve

Explanation:

The location of aortic valve i.e between the aorta and left ventricle,  allows it to stop the blood from flowing back into the left ventricle.

The other options are incorrect as -  

Pulmonary valve being located between the pulmonary artery and right ventricle, stop the blood from flowing back into the right ventricle.

The tricuspid valve and mitral valve stops the blood from flowing back into the right atria and left atria respectively.  

Hence, option D is correct.

p waves. These waves are of a higher frequency than surface

waves .