The correct answer is option 4, the left ventricle.

To understand why the left ventricle has the thickest muscular walls, we need to understand the function of each chamber of the heart. The heart has four chambers: two atria (left atrium and right atrium) and two ventricles (left ventricle and right ventricle).

The atria receive blood returning to the heart, and the ventricles pump blood out of the heart to the rest of the body. The left atrium receives oxygenated blood from the lungs and transfers it to the left ventricle. The left ventricle then pumps this oxygen-rich blood into the main artery of the body, the aorta, and distributes it to the rest of the body.

The left ventricle has the thickest muscular walls because it needs to generate enough force to pump the blood throughout the entire body. It has to overcome the greatest amount of resistance compared to the other chambers since it is responsible for supplying oxygenated blood to all the organs and tissues. The thick muscular walls of the left ventricle allow it to contract strongly and produce enough pressure to generate this forceful pumping action.

Therefore, the correct answer is the left ventricle.