Reaching a Nash Equilibrium Means That

Reaching a Nash equilibrium means that:

A) the outcome will be positive-positive.
B) a cooperative equilibrium has been reached.
C) the players have reached a stable outcome where neither would wish to change his strategy once he finds out what the other player is doing.
D) the players have failed to reach a stable outcome because one player will always wish to change his strategy once he finds out what the other player is doing.