Identify the correct reason for the second node A. Reflection

B. Definition of midpoint

C. Definition of congruent segments

I think your teacher meant to say Reflexive property instead of reflection. The reflexive property is the idea that any segment is the same length as itself. It seems like a trivial idea, but it's useful to be able to pull triangles apart like in this problem here.

Note: when naming a segment, the order of the endpoints listed doesn't matter. So that's why EU is the same as UE.