Answer: choice 3) Complementary angles
Because angle ACD is 90 degrees (from the square marker symbol), this means that angle BCD is 90 degrees. The two angles add to 180 to form a straight line so 90+90 = 180. Now that we know angle BCD is 90 degrees, this means that breaking up this angle will have the sub-angles add back up to 90. Therefore, angle DCE adds to angle BCE to get 90 degrees. This is why the two angles are complementary.
