Which of the following describes two angles that add up to 90 degrees?

The correct choice describes two angles that add up to 90 degrees as complementary angles. This mathematical term specifically defines the relationship between two angles when their measures sum to exactly 90 degrees. For example, if one angle measures 30 degrees, its complement would measure 60 degrees, since 30 + 60 = 90.

To understand why this term is appropriate, it's essential to recognize what other terms refer to. Supplementary angles, for instance, are defined as two angles whose measures add up to 180 degrees. Linear angles refer to angles that are formed when two lines intersect, which create adjacent angles that sum to 180 degrees. Adjacent angles are angles that share a common vertex and a side but do not overlap; they can be complementary, supplementary, or neither based on their measures.

Thus, unique to complementary angles is the specific condition of their sums equaling 90 degrees, solidifying this as the correct answer.