In this scenario, Lexi started with the line segment \(CD\) and folded the paper such that point \(C\) was on top of point \(D\). The point of intersection of the fold and the line segment \(CD\) is labeled as point \(E\). By folding \(C\) on top of \(D\), it implies that point \(E\) is the midpoint of segment \(CD\), making the fold the perpendicular bisector of line segment \(CD\).
After unfolding, Lexi draws a line \(QR\) through point \(E\), and then lines \(CR\) and \(DR\) through points \(C, R\) and \(D, R\) respectively.
Since point \(E\) is the midpoint of segment \(CD\) due to the folding, and lines \(CR\) and \(DR\) are derived from points \(C\) and \(D\) respectively stretching through point \(R\), the line segment or line that acts as the perpendicular bisector of \(CD\) in this construction is the fold itself.
Therefore, the correct answer in terms of bisector property is:
\(CD\) (with a bar over it, indicating it is a segment).