Why C? And why not the others?

Correct answer: C. The sinoatrial (SA) node

The sinoatrial node, a small patch of specialized, spontaneously firing cells in the wall of the right atrium near where the vena cava enters, generates the electrical impulse that starts every normal heartbeat, earning it the nickname the heart's natural pacemaker; the autonomic nervous system adjusts how fast it fires but does not itself originate the signal. The atrioventricular node's job comes right after: it receives that signal from the atria and briefly delays it, giving the atria time to finish contracting before the ventricles are triggered, but it does not start the impulse itself. The bundle of His and the Purkinje fibers sit further downstream still, carrying the already-initiated signal down the septum and out through the ventricular walls so the lower chambers contract in a coordinated wave, relay structures rather than the impulse's point of origin.

Source: Encyclopaedia Britannica: sinoatrial node