The heart's internal pacemaker beats by:

an internal implant that sends an electrical impulse through the heart

the excitation of cardiac muscle cells at the sinoatrial node followed by the atrioventricular node

the excitation of cardiac muscle cells at the atrioventricular node followed by the sinoatrial node

the action of the sinus

Answer: Option B.

Explanation:

The heart's internal pacemaker beats due to the excitation of cardiac muscle cells at sinoatrial node which are followed by atrioventricular node.

An electrical signal is received by heart from the sinoatrial node which triggers the contraction of cardiac muscle cells in the atria. In systolic Phase, the signal produced get pauses at the atrioventricular node before reaching to the walls of ventricles that allows the blood to get pumped through the body. The heart then allows to relax in the diastole and blood fills again following the same process.

Hence, the correct option is B.