Muscular wall of the heart is externally covered by

The correct answer is C. Epicardium.

The epicardium is the outermost layer of the heart. It is a thin, serous membrane that is continuous with the visceral pericardium, which is the outer layer of the pericardial sac. The epicardium is composed of a single layer of flattened mesothelial cells that are surrounded by a thin layer of connective tissue. The epicardium provides a smooth, slippery surface that allows the heart to move freely within the pericardial sac. It also helps to protect the heart from injury and infection.

The other options are incorrect because:

• Epithelium is a type of tissue that lines the surfaces of the body and organs. It is not a layer of the heart.
• Endothelium is a type of tissue that lines the blood vessels and heart chambers. It is not a layer of the heart.
• Myocardium is the muscular layer of the heart. It is not the outermost layer of the heart.