Which of the Following Defines the Nash Equilibrium

Which of the following defines the Nash equilibrium?

A) where the output chosen by each firm are consistent with each other
B) the position resulting from everyone making their optimal decision based on their assumptions about their rivals' decisions
C) where two or more firms (or people) by attempting independently to choose the best strategy for whatever the other(s) are likely to do, end up in a worse position than if they had co- operated in the first place.
D) where neither firm is able to earn supernormal profits.