Ans: C
Sketch the figure according to the question.
Since $PQRS$ is a rhombus, then $PQ = QR$, $ST = QT$ and $\angle QTR =90^\circ$. Hence, we have
$\begin{array}{rcl}
\dfrac{PQ}{ST} & = & \dfrac{QR}{QT} \\
\dfrac{PQ}{ST} & = & \dfrac{1}{\sin \theta}
\end{array}$
