Ans: A
I must be true. Since $p$, $q$, $r$ and $s$ is a geometric sequence, then we have
$\begin{array}{rcl}
\dfrac{q}{p} & = & \dfrac{s}{r} \\
ps & = & qr
\end{array}$
II may not be true. For the geometric sequence $p=1$, $q=2$, $r=4$ and $s=8$, it is not true that $p+s=q+r$.
III may not be true. For the geometric sequence $p=1$, $q=\dfrac{1}{2}$, $r=\dfrac{1}{4}$ and $s=\dfrac{1}{8}$, it is not true that $p< q< r< s$.
