Question

Fourth geometry question

Answer 1

Δ DEC is a reflection of ΔABC across line k.

Answer 2

Lines DEC are a Reflection of lines ABC and the way you van tell is you visualize in your mind flipping the image to the right or to the left it doesn't matter but if they line up then that is how you know it is a reflection.
I hope this helps a lot and have a good day.