Final answer:
The sum of measures of two angles is 90° when they are complementary angles, which means they add up to 90°.
Step-by-step explanation:
If the sum of measures of two angles is 90°, then the angles are complementary angles. Complementary angles are two angles whose measures add up to 90°.
This means that each of the two angles complements the other to reach the total of 90°. An everyday example of complementary angles is the hands of a clock when they form a right angle at 3 o'clock or 9 o'clock. The two hands at that time are not in the same direction or pointing opposite directions, and they certainly do not form a 270° angle between each other. Therefore, complementary angles is the correct answer to fill in the blank.