The correct option is A Both assertion and reason, are true and the reason is the correct explanation of assertion.
Sinu-auricular node or S-A node is located in the wall of the right auricle below the opening of superior vena cava. The S-A node has a unique property of self-excitation, which enables it to act as "pacemaker" of the heart. It spontaneously initiates a wave of contraction which spreads over both the auricles more or less simultaneously along the muscle fibres that fan out from the pacemaker.