Question

triangle def is congruent to △def by the sss theorem. which single rigid transformation is required to map △def onto △def? dilation reflection rotation translation

Explanation:

Dilation changes the size and is not a rigid - transformation. Translation moves the figure without flipping or rotating it. Rotation turns the figure around a point. Reflection flips the figure over a line. Since the orientation of $\triangle DEF$ and $\triangle D'EF$ seems to be mirror - like (judging from the general appearance of the triangles in the image), a reflection is the single rigid transformation that can map one onto the other.

Answer:

reflection