All three of the endings were flawed.
Option B just makes no sense whatsoever - Franklin killing his friend and mentor because some rich guy tells him to?? I think not.
Option A is disappointing because, as much as Trevor's demise is poignant and kind of makes sense:
a) It feels very out of character for Franklin
b) You get no resolution to any of the other storylines.
Option C is just the Wayne's World "Super Mega Happy Ending". Which feels a bit cheap in of itself, but also fails to resolve the biggest plotline of the story i.e. Michael vs. Trevor.
Remember, Michael betrayed Trevor ten years ago and arranged for him to be killed. He did so because Trevor was a crazy lunatic who cared about nothing except the chase, and who was never going to let Michael stop the life of crime until it killed him.
And at the end of ending C, that still hasn't been resolved. And you have to believe that, after the end of the game, history is likely to just repeat itself. One or both of them is inevitably going to end up dead, and you're just left not knowing who or how.
What was needed was a final mission which pitted Michael and Trevor against each other and let one of them resolve the storyline. Rather than arbitrarily putting the gun in Franklin's hands.
Don't get me wrong - I loved the story. In general, the best of the series by miles.
But the endings were disappointing.