The correct answer is a) myocardium.
The myocardium is the middle layer of the heart wall and is primarily composed of cardiac muscle tissue. This layer contains myofibrils, which are made up of contractile proteins, actin and myosin, allowing the heart muscles to contract and pump blood. Additionally, the myocardium features intercalated discs, which are specialized structures that connect individual heart muscle cells (cardiomyocytes) to each other. These discs facilitate communication between cells and ensure synchronized contractions of the heart. In contrast, the endocardium is the inner lining of the heart, and the epicardium is the outer layer, while the pericardium is the fibrous sac surrounding the heart.