A math teacher shows a triangle with all angles less than 90 degrees. What type of triangle is she showing?

The triangle described in the question features all angles measuring less than 90 degrees. This characteristic defines an acute triangle, which is precisely where all interior angles are acute. In geometry, triangles are categorized based on their angles, and an acute triangle is one of the primary types recognized.

In contrast, an isosceles triangle refers to a triangle with at least two sides of equal length, and it can be either acute, obtuse, or right-angled. A scalene triangle is defined as a triangle with all sides of different lengths, and like the isosceles triangle, it can be either acute, obtuse, or right. An obtuse triangle, however, has one angle that measures more than 90 degrees, which contradicts the description of a triangle where all angles are less than 90 degrees.

Therefore, the correct understanding of the triangle with all acute angles clearly identifies it as an acute triangle, which is why B is the accurate choice.