SInce the square that underwent a dilation had a perimeter of 200 cm, that means that each side had a length of 200/4 = 50 cm (recall that all sides of a square are equal in size)
Then from being 50 cm long, the sides were converted to a square of sides 40 cm long. That means that the dilation converted 50 into 40. which means that each side was multiplied by 4 and divided by 5:
50 * 4 / 5 = 200 / 5 = 40
Then the SCALE factor of the dilation was 4 / 5 = 0.8
Through a dilation , eaxh point (x, y) is transformed via the product of the coordinates by the scale factor. Then for our case:
(x, y ) ---> (0.8 x, 0.8 y)
which agrees with the last option they give you (option D)