The correct answer is option 'D', which is the left ventricle.

The heart is a vital organ in the circulatory system that pumps blood throughout the body. It consists of four chambers: two atria (left and right) and two ventricles (left and right). The atria receive blood from the veins, while the ventricles pump blood out of the heart.

Here is an explanation of why the left ventricle is responsible for pushing blood into systemic circulation:

1. Function of the ventricles:
The ventricles are the lower chambers of the heart and are responsible for pumping blood out to the body. They have thicker muscular walls compared to the atria because they need to generate enough force to propel blood through the entire circulatory system.

2. Pulmonary circulation:
Before blood is pumped into systemic circulation, it goes through another circulation called pulmonary circulation. The right ventricle is responsible for pumping blood to the lungs for oxygenation. Oxygen-depleted blood from the body enters the right atrium, passes through the tricuspid valve, and enters the right ventricle. The right ventricle then contracts and pushes the deoxygenated blood through the pulmonary valve into the pulmonary artery, leading to the lungs.

3. Systemic circulation:
After the blood is oxygenated in the lungs, it returns to the heart and enters the left atrium. From the left atrium, the blood passes through the mitral valve and enters the left ventricle. The left ventricle then contracts forcefully, generating pressure to push the oxygenated blood out of the heart through the aortic valve and into the aorta, which is the main artery of the body. From the aorta, the blood is distributed to all the organs and tissues through a network of arteries, arterioles, and eventually, capillaries.

4. Importance of the left ventricle:
The left ventricle is responsible for the systemic circulation, which supplies oxygenated blood to all the organs and tissues of the body. Its muscular walls are thicker compared to the right ventricle because it needs to generate a higher pressure to overcome the resistance in the systemic circulation.

In summary, the left ventricle is the chamber of the heart that pushes blood into systemic circulation. It receives oxygenated blood from the left atrium and contracts forcefully to propel the blood through the aorta and into the arteries, delivering oxygen and nutrients to the body's tissues and organs.