In a parallelogram, the diagonals bisect each other. This means that the segments formed by the intersection of the diagonals are congruent.
Given that segment \( SP \) is one part of diagonal \( QS \) that connects vertices \( Q \) and \( S \), the segment that must be congruent to \( SP \) is the part of the diagonal \( PR \), which is the other segment created by the intersection of the diagonals \( QT \) and \( SR \).
Thus, the segment of the figure that must be congruent to segment \( SP \) is \( PR \).
Final answer: PR