I firmly believe there is no such thing as a canon ending in this game, but if I had to choose one it would be C.
I've explained why several times, but to recap:
- There are post-story missions for Michael and Trevor (Mrs Philips and Epsilon missions - not sure if the Epsilon missions can be done before the story is complete, but I recall reading they're only available after the story).
- Most people will have become attached to all of the characters in one way or another. Losing one would remove something meaningful from the game.
- It's really only possible to buy all of the properties after the story... so, what happens to the Michael and Trevor-specific properties if they're dead? They can't be purchased. Oops.
- Goto 10.
C. It's the only option that makes sense.