Which triangle has one angle that is 90 degrees?

A triangle that has one angle measuring exactly 90 degrees is classified as a right triangle. This type of triangle is defined by the presence of a right angle, which is a crucial characteristic that distinguishes it from other types of triangles. In a right triangle, the side opposite the right angle is known as the hypotenuse, and it forms the foundation for various mathematical concepts, including the Pythagorean theorem.

In contrast, other types of triangles do not contain a right angle. An obtuse triangle has one angle greater than 90 degrees, while an acute triangle has all angles less than 90 degrees. A scalene triangle, on the other hand, specifically refers to a triangle where all sides and angles are of different lengths and measures, but does not inherently define the angles. Therefore, the defining characteristic of a right triangle is its inclusion of a 90-degree angle, making it the correct choice in this context.