Right answer is (b) sino-atrial node
The best explanation: Located in the top right atrium near the entry of the vena cava, are a group of cells known as the sion-atrial node (SA node) that initiates the heart activity. Because of this is also considered as the primary pacemaker of the heart. The SA node is 25 to 30 mm in length and 2 to 5 mm in thickness.
