The magnetic chessboard is placed vertically, and the chess pieces are placed on it to be subjected to gravity, support, attraction and friction; Gravity is vertical downward and friction is vertical upward. Gravity and friction are a pair of balancing forces, not a pair of interaction forces. Therefore, A is right and C is wrong;
The reason why the chess pieces don't slide down is because of the vertical friction of the chessboard, BD is wrong;
So choose: a.