The answer is the 1st graph. This is because a square have 4 sides and it is all the same. on the x axis (Side length) when it is 4 the perimeter (Length around the outside) will be 4 x 4 which is 16.
The volume becomes 1/3 of the original. With a rectangular base, volume of a pyramid is V=(length*width*height)/3. Cutting the height to 1/3 of the original length will make the total volume 1/3 of the original volume.