Derek Is Asked to Participate as a Speaker at an Official

Derek is asked to participate as a speaker at an official event. A few days before the program, Derek decides to write a message declining the request. Which of the following sentences should Derek use to end his message?

A) I hope the event turns out to be a great success.
B) It is with regret that I must refuse your request.
C) I hope that you understand why I made this decision.
D) Perhaps next time I will not have to refuse such an offer.
E) As you know, I have valid reasons for attending this event.