The diagram given here is the standard ECG of a normal person. The P-wave represents the

 

‘P’ wave in ECG represent atrial depolarisation which leads to atrial systole (contraction). QRS complex represents ventricular depolarisation and ventricular contraction begins shortly after ‘Q’. ‘T’ wave represents the ventricular repolarisation. The end of ‘T’ wave marks the end of ventricular systole.

The option ‘beginning of the systole’ should be considered incorrect. The term ‘systole’ (not preceded by atrial) is conventionally used to refer to ventricular systole. Moreover, the contraction of atria starts somewhere at the peak of the ‘P’ wave.