The five actions split naturally into two phases of air travel: things done before arriving at the airport (choosing the flight and paying for it), and things done at or after arriving (printing, boarding, flying).
- Choose (B) and Pay (A): These must come first, and in that order, since you choose a flight before you can pay for it.
- Print (C): This comes right after payment, since the ticket or boarding pass can only be printed once the booking is confirmed and paid for.
- Board (E): This follows printing, since the printed ticket or boarding pass is needed to board.
- Fly (D): This is always the final action, only happening once you are already on board.
Putting the two phases together in order gives Choose, Pay, Print, Board, Fly, or B, A, C, E, D.
So the correct answer is BACED.