A rectangular block of wood rests on the ground as shown in the diagram below.
Which of the following is the easiest way for a force F to topple the block?
For each block we identify the pivot point and the perpendicular distance of force F from the pivot point.
We can see that the perpendicular distance for D is the greatest. Force F will produce the greatest moment for D, hence it will topple the easiest.
Answer is D.