The correct answer is endocardium.
The endocardium is termed as thin and smooth and is the innermost layer of the heart. Endocardium tissues are biologically and embryologically similar to endothelial cells which lines blood vessels.
It provides protection to valves and heart chambers. It is the muscular tissue which is responsible for contraction of the heart.
It also controls myocardial function. It acts as kind of blood-heart barrier and controls ionic composition of extracellular fluid where cardiomyocytes bathe.