Which of the Following Sentences Most Effectively Implies the Refusal

Multiple Choice

Which of the following sentences most effectively implies the refusal?

A) I'm sorry that I won't be able to speak at your annual conference,but I'd like to be considered in future years.
B) Although I'll be in Southeast Asia on business on the date of your annual conference,I could speak to members prior to that date.
C) Although I am unavailable to speak at your annual conference,I can recommend another speaker.
D) Thank you for your kind invitation to speak at this year's annual conference.