The Answer should be option D i.e 400
This is because after the last fold if the two equal sides measure 10cm each then the length of the base of the triangle before the last fold (i.e the base of the last triangle shown in the figure) should be 2 * 10 cm i.e 20 cm. Hence the base of the square piece of paper is 20 cm. Hence the area is $20^{2}$ which is 400.