Step 1
Going through all the options, in the dilation triangle ABC will be dilated with a scale factor to get triangle A'B'C'. The question clearly stated that triangle ABC is congruent to triangle DEF. Congruent means that both triangles are identical in form. So if a scale factor via dilation is applied to either of the triangles then 1 triangle will be bigger or smaller than the other triangle and the condition of congruency will fail as they will no more be identical. Therefore, the transformation that cannot be used to prove that triangle ABC is congruent to triangle DEF is dilation.
Hence the answer is option A