The correct answer is option 1, pericardium. The pericardium is a protective sac that encloses the human heart. It is made up of two layers: the outer fibrous pericardium and the inner serous pericardium. The fibrous pericardium is a tough, fibrous layer that helps protect the heart and anchors it to surrounding structures. Inside the fibrous pericardium is the serous pericardium, which consists of two layers - the parietal layer, which lines the fibrous pericardium, and the visceral layer (also known as the epicardium), which covers the surface of the heart. The pericardium acts as a lubricating membrane, allowing the heart to move and beat within the chest without friction. It also helps to prevent infection and keeps the heart in place within the chest cavity.