Answer:
Complementary
Explanation:
If the exterior sides of two adjacent angles are perpendicular, then the angles are complementary angles.
The statement is defining two adjacent triangles that are on a right angles, that is, a angle equal to 90°. We can deduct this because they problem states that their exterior sides are perpendicular, these sides are forming a right angle, because that's what perpendicularity means.
Therefore, the right term that completes the statement is "complementary", because complementary angles are define as adjacent angles which sum is 90°.