A reflection over diagonal RT
1) Considering that we're dealing with reflection symmetry and the Parallelogram QRST.
(In red the axis of symmetry)
2) So we can reflect that quadrilateral QRST onto itself by those two red lines of symmetry, its line half of that figure is mirrored across.
3) Hence, examining the answers, we can state that the answer is
A reflection over diagonal RT
