To classify triangles by angles, you need to consider the measures of their angles.
An acute triangle is a triangle in which all three angles are less than 90 degrees. In other words, all angles of an acute triangle are "small" angles.
A right triangle is a triangle that has one angle measuring exactly 90 degrees. This angle is called a right angle.
An obtuse triangle is a triangle that has one angle measuring more than 90 degrees. In other words, it has one "big" angle.
The correct answer "acute, right, obtuse" represents the classifications of triangles based on their angles because it includes all the possibilities. It covers all cases where a triangle can have three different types of angles: all small angles (acute), one right angle (right), or one big angle (obtuse).