The correct answer is a) Ca++
Cardio-vascular physiology is somewhat different from the physiology of rest of the organs of the body. Heart contains two nodes, namely Sino-atrial (SA) node and Atrio-ventricular (AV) node. These node points are the actual points where nerve impulse is generated in the heart; which is actually responsible for our heart beat.
Cells/cardio-myocytes of SA and AV node do not have a true action potential. In these, action potential is generated by Ca++ and K+ ions, unlike nerve and muscle cells, where action potential is generated by Na+ ions. The role of Na+ ions is very less in case of action potential of heart cells.
One important thing to note is that movement of Ca++ currents is slow in comparison to Na+ currents which are fast acting.