Which characteristic distinguishes an isosceles trapezoid from other trapezoids?

An isosceles trapezoid is specifically characterized by having its non-parallel sides, or legs, being congruent. This is the defining feature that sets it apart from other trapezoids, which may have sides of different lengths. In an isosceles trapezoid, the congruence of the non-parallel sides leads to a number of other properties, such as the base angles being equal.

The other characteristics listed in the choices do not accurately define what makes an isosceles trapezoid unique. For example, while all trapezoids have one pair of parallel sides, this characteristic is not distinguishing. Moreover, angles in trapezoids can vary; they are not required to be all acute nor is it a requirement that only one angle be right. Thus, the congruence of the non-parallel sides is the key trait that identifies an isosceles trapezoid.