Sticking a magnet on the weight is equivalent to changing the weight of the weight. The greater the weight, the smaller the moment arm, and the smaller the number read from the weight, which will make the measured value less than the real mass of the object. Therefore, A and D are wrong and B is correct;
When you lift the button with B, the fulcrum position changes, which is equivalent to the object's arm getting bigger, and the maximum scale value of the ruler is smaller than when you lift the button with A, so C is wrong.
So choose B.