When triangle ABC is reflected across the y-axis to produce triangle DEF, the coordinates of each point in triangle ABC change based on their original position. If point A has coordinates \((x_A, y_A)\), after reflection across the y-axis, point D will have coordinates \((-x_A, y_A)\). Similarly, points B and C will be reflected to points E and F with coordinates \((-x_B, y_B)\) and \((-x_C, y_C)\), respectively.
To determine which segment in triangle DEF is congruent to segment BC, we note that:
- Segment BC connects points B and C (so, it is represented by the distance between points B and C).
- The segment DE connects points D and E (where points D and E are the reflections of points A and B across the y-axis).
The properties of reflections tell us that the lengths of corresponding segments remain the same after a reflection. Therefore, the segment DE in triangle DEF is congruent to segment BC in triangle ABC, as both segments have the same length but are oriented differently.
Thus, the answer is: Segment DE on triangle DEF is congruent to segment BC on triangle ABC.