The correct answer is a. The left ventricle is responsible for securing the flow of blood into the systemic circuit, meaning around the entire body. On the other hand, the right ventricle is responsible for securing the flow of blood only through the lungs (pulmonary circuit). Therefore, the left ventricle has a more demanding task, and needs a stronger muscular system to power the pumping of the blood around the human body. That is why the left ventricle has a thicker muscle wall.