Which of the following regulates the heart's contractility?

The layer of the heart that regulates its contractility is the myocardium. This is the muscular layer of the heart responsible for the contraction and pumping of blood. The myocardium contains cardiac muscle fibers that allow the heart to contract and generate force necessary to circulate blood throughout the body.

In contrast, the epicardium is the outer layer of the heart that provides a protective covering and is involved in the production of pericardial fluid, which lubricates the heart, rather than directly influencing contractility. The endocardium lines the heart chambers and helps ensure smooth blood flow, but it does not actively contribute to muscle contraction. Similarly, the pericardium is a fibrous sac surrounding the heart that helps protect it and anchor it in place, but it does not play a role in contractility either.

Thus, the correct answer focuses on the myocardium as the key component that controls the heart's ability to contract effectively.