In this scenario, Lexi folded the paper so that point C was on top of point D, which means that the fold is the perpendicular bisector of the line segment CD. When she unfolded the paper, point E is where the fold intersects the line segment CD.
She then drew a line QR through point E at an arbitrary angle, and also drew lines CR and DR through points C and R, and points D and R respectively.
Since E is the intersection point where the paper was folded (the perpendicular bisector of segment CD), the segment bisector of CD is indeed the line that runs through E, which is the line segment QR.
Therefore, the segment bisector in this scenario is:
• ⟷QR