Let us join AC and BD.

In ΔABC,

P and Q are the mid-points of AB and BC respectively

PQ || AC and PQ = AC (Mid-point theorem) (1)

Similarly in ΔADC,

SR || AC and SR = AC (Mid-point theorem) (2)

Clearly,

PQ || SR and PQ = SR

Since in quadrilateral PQRS, one pair of opposite sides is equal and parallel to each other, it is a parallelogram

PS || QR and PS = QR (Opposite sides of parallelogram) (3)

In ΔBCD, Q and R are the mid-points of side BC and CD respectively.

QR || BD and QR = BD (Mid-point theorem) (4)

However, the diagonals of a rectangle are equal.

AC = BD (5)

By using equation (1), (2), (3), (4), and (5), we obtain

PQ = QR = SR = PS

Therefore, PQRS is a rhombus