C

After the ‘a’ wave (right-atrial contraction) the next event is ventricular depolarisation (R wave), which is promptly followed by AV-valve closure producing the first heart sound (S1). Early systole then enters the rapid ejection phase, and ventricular repolarisation (T wave) occurs last near end-systole. Hence the temporal order is R wave → S1 → rapid ejection → T wave (C-A-D-B) [Eyster 1911 PMID 19867499; Devine 2007 PMID 17943049].