The wall of the heart is made up of
A. Epicardium
B. Myocardium
C. Endocardium
D. All of the above

Hint: The heart is the muscular organ which helps in the pumping of blood in most of the animals. The pumping of blood carries oxygen and various other nutrients to the body while it also carries the metabolic wastes away from the lungs.

Complete step-by-step answer:
The heart is right in the centre between the two lungs and above the diaphragm. The heart in adult humans is about the size of a closed fist of about 12 cm in length and 9 cm in width. The heart is divided into 4 chambers: the upper left and right atria and the lower left and right ventricles. It is protected by a double-walled membranous covering called the pericardium. It contains lubricating pericardial fluid which reduces friction during a heartbeat and protects it from mechanical injuries. The wall of the heart is divided into three layers: the inner layer endocardium, the middle layer myocardium and the outer layer epicardium.
The atria have a thinner wall because their main function is to receive blood from the body and pump it into the very next ventricles. The ventricles have thick muscular walls as they have to pump blood to long distances. The right ventricles pump blood only up to the lungs for oxygenation while the left ventricles pump it to the farthest point in the body like the toes in the feet or up to the brain against the gravity.
So, the wall of the heart is made up of epicardium, myocardium and endocardium.

Note: The narrow end of the approximately triangular heart is pointed to the left side and during working the contraction of the heart is most powerful at this end which gives a feeling that it is on the left side.