Answer:
The triangle ABC have a 90 degree angle, therefore the acute angles in A and B are complementary, if you add them the result is equal to 90 degree.
Explanation:
Supplementary angles are the angles that you can add and the result will be equal to 180 degrees. While the complementary angles are the ones that the addition result is 90 degrees. From your figure the addition of the angles ADC and BDC should be 180, therefore they are supplementary angles, while angles A and B are complementary, their sum should be 90.