P in the heart have thicker muscular walls than Q do. Also, R ensures that blood does not flow backwards when the atria or ventricles contract. P is ventricles, Q is atria, and R is valves.

The correct option is A.

P: ventricles

Q: atria

R: valves

The heart consists of four chambers: right and left atria above, and right and left ventricles below. The right atrium connects to the right ventricle via the tricuspid valve, while the left atrium connects to the left ventricle via the bicuspid (mitral) valve. These valves prevent the backflow of blood into the atria when the ventricles contract.

Ventricles have thicker muscular walls than atria. This is because when each atrium contracts, it only needs to propel the blood a short distance into each ventricle. In contrast, the ventricles require a more powerful contraction to propel blood into the systemic circulation.

Therefore:

• P is ventricles
• Q is atria
• R is valves