Only the player who is currently taking their turn can win the game, so player C would be the winner.

Official Rule:

> ENDING THE GAME
If you have—or reach—10 victory
points on your turn, the game ends
immediately and you win! You can only
win during your turn. If somehow you
find you have 10 victory points during
another player’s turn, you must wait
until your next turn to claim victory.