The correct answer is a. valves in the heart.
Explanation:
Valves in the heart are designed to ensure one-way blood flow, preventing backflow when the chambers contract. When a chamber contracts, the valves close to prevent blood from flowing back into the previous chamber.
Why other options are incorrect:
b. thick muscular walls of ventricles:
While the thick muscular walls of the ventricles help generate the force needed to pump blood out of the heart, they are not responsible for preventing backflow. The valves are the primary mechanism for preventing backflow.
c. thin walls of atria:
Thin walls of atria are not relevant to preventing backflow. The atria are the receiving chambers of the heart, and their thin walls facilitate the easy inflow of blood.
d. all of the above:
Only the valves are responsible for preventing backflow. The thick muscular walls of ventricles help with pumping, but not preventing backflow. Thin atria walls are also not relevant to backflow prevention.