Final answer:
The innermost layer of the heart is the endocardium. It is the layer that covers the heart chambers and valves and is distinct from the epicardium and myocardium, which are the outer and middle layers respectively.
Step-by-step explanation:
The innermost layer of the heart is called the endocardium. This layer covers the heart chambers and valves. There are three layers that make up the heart wall: the epicardium (outermost layer), the myocardium (middle layer consisting of heart muscle cells), and the endocardium (innermost layer). Moreover, the epicardium is also known as the visceral pericardium, as part of the pericardium, which is a membrane structure that surrounds and protects the heart.
Therefore, the answer to the question is option 2) endocardium.