In a parallelogram, the diagonals bisect each other. This means that the segments created by the diagonals, when they intersect at point P, are congruent.
Since SP is one of the segments formed by one of the diagonals (QS and RT), the segment that must be congruent to segment SP is segment PT.
Therefore, the correct response is:
PT¯¯¯¯¯¯¯ upper P upper T