Again, I got this off from Google. So the rule of thumb here is:
The nearer the light is to the object, the shorter the shadow will be.

If you apply this theory, you will realise that only 1 and 3 makes sense as point B must have the shortest shadow length. However 3 is wrong B cannot be 0 length. So long as the object is not flat on the floor, there is definitely a shadow. Hence, only option 1 is correct. Hence, 1 is the answer.

Hope you understand.