Conduction system of the heart contains autorhythmic fibres which are usually Nodes of the heart. There are two types of nodes-Sinoatrial node and the atrioventricular node.
The sinoatrial node is known as the pacemaker of the heart as it maintains the pace of the heartbeat due to its efficiency if higher depolarization than other node and conduction cells like a bundle of His and Purkinje fibres.
Since depolarization causes a contraction in the muscle cells, therefore, heartbeats and thus option-D is the correct answer.
