Which type of triangle has all angles less than 90 degrees?

An acute triangle is defined as a triangle in which all three internal angles are less than 90 degrees. This characteristic sets it apart from other types of triangles. In an acute triangle, because each angle is less than 90 degrees, the sum of the angles remains equal to 180 degrees, which is a fundamental property of all triangles.

In contrast, obtuse triangles contain one angle that is greater than 90 degrees, while right triangles have one angle equal to 90 degrees. Scalene triangles describe triangles where all sides and angles are different; this term does not specify the size of the angles, meaning the angles could be acute, right, or obtuse. Thus, the specific definition of an acute triangle as having all angles less than 90 degrees makes it the correct answer.