Sentence A could also mean that the competition has not yet happened, but it is possible to win if you try. I probably would consider sentence C improper because it implies that you won (past tense: were able) but then says 'if you tried' so it also implies that you may not have tried yet. I would not use sentence C. Sorry so wordy and confusing. What you have written sentence D is correct. You won in that instance.

possibilities: You could win if you tried = You can win, you just need to try You were able to win if you had only tried. (you could have won, but you didn't try!) You (only) won because you tried. You won because you tried.