(a) Rhombus.
In ΔBDC, Q and R are the mid-points of BD and CD respectively.
∴ QR || BC and QR = \(\frac12\) BC
Similarly, in ΔABC, PS || BC and PS = \(\frac12\) BC =
∴ PS = QR
Also in ΔABD, PQ || AD and PQ = \(\frac12\) AD
In ΔADC, SR || AD and SR = \(\frac12\) AD =
∴ PQ = SR
AD = BC ⇒ PS = QR = PQ = SR ⇒ PQRS is a rhombus.