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