Which portion of the heart wall is responsible for the pumping action?

The heart is composed of three layers: (1) pericardium, (2) myocardium, and (3) endocardium.

The correct answer for this question is the myocardium. The myocardium is composed of cardiac muscle tissue that responds to the pacemaker activity of the heart (originates from the sinuatrial node) and performs rhythmic contractions that corresponds to a person's heartbeat.

The pericardium forms the pericardial sac wherein the pericardial fluid is located. The endocardium lines the atria and ventricles as well as contributes to the formation of the heart valves.