The correct option is
B Ventricular contraction
An electrocardiogram (EKG or ECG) is a test that measures the electrical activity of the heartbeat. With each beat, an electrical impulse (or wave) travels through the heart. This wave causes the muscle to squeeze and pump blood from the heart. A normal heartbeat on ECG will show the timing of the top and lower chambers. The right and left atria or upper chambers make the first wave called a P wave" following a flat line when the electrical impulse goes to the ventricles. The right and left ventricles make the next wave called a QRS complex." The final wave or T wave represents electrical recovery or return to a resting state for the ventricles
So, the correct answer is 'Ventricular contraction'