Can there be two winners at the same time in Catan?
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.


Source: Downloads page on www.catan.com