The correct option is C Contraction of both the atria
In the given diagram the P-wave represents the electrical excitation (or depolarization) of the atria, which leads to the contraction of both the atria. The QRS complex represents the depolarization of the ventricles, which initiates the ventricular contraction. The contraction starts shortly after Q and marks the beginning of the systole. The T-wave represents the return of the ventricles from excited to the normal state (repolarisation). The end of the T-wave marks the end of systole.