Which of the following statements is true about an angle that measures 180 degrees?

An angle that measures 180 degrees is classified as a straight angle. In the context of angle relationships, supplementary angles are defined as two angles whose measures add up to 180 degrees. Therefore, since an angle measuring exactly 180 degrees itself can pair with a 0-degree angle to meet this definition, it is indeed a supplementary angle.

In contrast, acute angles measure less than 90 degrees, obtuse angles measure greater than 90 degrees but less than 180 degrees, and right angles measure exactly 90 degrees. Since a 180-degree angle does not fit any of those definitions, the only accurate classification among the options given is that it is a supplementary angle. This highlights its role in geometric relationships involving angle sums.