The correct answer is: B. Endocardium
The endocardium is the innermost layer of the heart. It is a thin, smooth membrane that lines the inside of the heart chambers and valves. The endocardium is made up of a single layer of endothelial cells, which are cells that line the blood vessels. The endocardium is important for preventing blood from clotting and for helping the heart valves to function properly.
The epicardium is the outermost layer of the heart. It is a thin, serous membrane that covers the heart and the great vessels. The epicardium is continuous with the pericardium, which is a sac that surrounds the heart. The epicardium helps to protect the heart and to keep it in place.
The myocardium is the middle layer of the heart. It is a thick, muscular layer that makes up the bulk of the heart. The myocardium contracts to pump blood throughout the body. The myocardium is supplied with blood by the coronary arteries.
The pericardium is a sac that surrounds the heart. It is a double-layered membrane that helps to protect the heart and to keep it in place. The pericardium is filled with a small amount of fluid, which helps to reduce friction between the heart and the surrounding tissues.