Final answer:
Option (a), The pericardium is the thick fibrous sac that encloses the heart, consisting of two layers – the fibrous and serous pericardium – and serves to protect the heart and reduce friction.
Step-by-step explanation:
The thick fibrous sac that encloses the heart is called the pericardium. Specifically, the pericardium or pericardial sac is the membranous structure surrounding the heart, securing it within the thorax and reducing friction between the heart and surrounding tissues.
The pericardium consists of two layers: the fibrous pericardium, which is the tough outer layer, and the serous pericardium, which includes an inner visceral layer, also known as the epicardium, and an outer parietal layer that is fused to the fibrous pericardium. The heart also includes the endocardium, which lines the inner chambers, and the myocardium, the thick middle layer composed of cardiac muscle which facilitates the heart's contraction.