Which of the following triangles specifically has three angles measuring less than 90 degrees?

The correct answer is that an equilateral triangle specifically has three angles measuring less than 90 degrees. In an equilateral triangle, not only are all sides of equal length, but each of the internal angles measures exactly 60 degrees. This property ensures that all angles are acute, meaning they are each less than 90 degrees.

To contrast this with the other options, an obtuse triangle is defined by having one angle greater than 90 degrees, which does not meet the criteria specified in the question. A right triangle has one angle that is exactly 90 degrees, thus it also does not satisfy the requirement of having all angles less than 90 degrees. An isosceles triangle can be either acute, right, or obtuse depending on its construction, so it does not guarantee that all angles are less than 90 degrees. In summary, among these options, only the equilateral triangle consistently adheres to having all three angles measuring less than 90 degrees.