The correct option is B signals of contraction spread throughout ventricles
The QRS wave is considered as a single entity. Q is a small downward deflection, R is a large upright triangular wave, S is the ending downward wave. Impulses from the atrioventricular node travel through the Bundle of His and Purkinje fibres. Signals for contraction spread throughout the ventricles. The contraction starts shortly after Q and marks the beginning of contraction.
Signals for contraction spread throughout the atria during atrial contraction which gives rise to P waves.
T wave is caused due to ventricular relaxation (repolarization) and represents the return of the ventricles from excited to normal state.