Final answer:
The sinoatrial (SA) node (option 1) is the pacemaker of the heart, responsible for initiating the electrical signals that set the rhythm of the heartbeat.
Step-by-step explanation:
Under normal conditions, the pacemaker of the heart is the sinoatrial (SA) node. It is a specialized cluster of cells situated in the right atrium of the heart and holds the responsibility for initiating the electrical signals that regulate the heart's rhythm. This node generates action potentials that spread across the atria, leading to their contraction. Then these impulses reach the atrioventricular (AV) node, where they are briefly delayed before going through the atrioventricular bundle and the Purkinje fibers, eventually causing the ventricles to contract.