The fact that can be used to justify Step 1 is: A. AB = DE and segments with the same length are congruent.
This is because the fact that AB = DE implies that the two triangles are equal in length, which means that we can superimpose them onto each other such that A' = D' and B' = E'.
This is the foundation for the subsequent steps of the proof.
Therefore the fact that can be used to justify Step 1 is: A. AB = DE and segments with the same length are congruent.