Final answer:
The innermost layer of the heart, which is thin and smooth, is known as the endocardium. The option (B) is correct.
Step-by-step explanation:
The innermost layer of the heart, which is thin and smooth, is called the endocardium. The endocardium lines the heart chambers and valves and is composed primarily of simple epithelial cells which are also known as endothelium.
It is reinforced with a thin layer of connective tissue that binds it to the myocardium, the middle and thickest layer of the heart consisting of cardiac muscle. The other mentioned layers, the epicardium and the pericardium, serve as the outer protective layers of the heart with the pericardium making up the membranous sac that encloses the heart. Therefore, option (B) is correct.