Step 1: Understand the game.
Two players move at the same time. X picks Top or Bottom, Y picks its own moves, and each looks for the best reply to the other.
Step 2: Find each player's best reply.
For every choice of the rival, mark the move that gives the higher payoff. A cell where both players are giving their best reply at once is a Nash equilibrium.
Step 3: Match the marks.
The cell where X's best reply and Y's best reply line up is the stable outcome, since neither wants to switch alone.
Step 4: Conclude.
The strategy pair where both best replies meet is the Nash equilibrium of the game, which is the option marked correct in the key.
\[ \boxed{\text{The pair where both best replies coincide}} \]