Final answer:
Segment AC is parallel to segment DE if and only if triangle ABC is an isosceles triangle.
Step-by-step explanation:
In order for segment AC to be parallel to segment DE, the triangle ABC must be an isosceles triangle with congruent sides AB and BC. This means that points D and E must be the midpoints of sides AB and BC, respectively. Therefore, segment AC is parallel to segment DE if and only if triangle ABC is an isosceles triangle.