Ontario Mathematics Proficiency Practice Test

An equilateral triangle has the defining property that all its sides are equal in length. This characteristic leads to all internal angles also being equal, specifically each measuring 60 degrees. This uniformity in both side lengths and angles is fundamental to the definition of an equilateral triangle, differentiating it from other types of triangles.

In the context of the other options, they describe properties that do not fit the definition of an equilateral triangle. For instance, stating that all angles are right angles refers to a right triangle, while noting that all sides are unequal directly contradicts the very definition of an equilateral triangle. Similarly, mentioning an obtuse angle implies a triangle that is not equilateral because an equilateral triangle, by definition, cannot possess angles greater than 60 degrees. Therefore, the characteristic of having all sides equal is essential to understanding what qualifies a triangle as equilateral.