b
The balancing condition for Wheatstone's bridge is, $\frac{P}{Q}=\frac{S}{R}$
$\frac{P}{Q}=\frac{625}{R}$$.....................(1)$
When $\mathrm{P}$ and $\mathrm{Q}$ are interchanged we get,
$\frac{Q}{P}=\frac{676}{R}$
$\frac{P}{Q}=\frac{R}{676}$$.....................(2)$
Equating both the equations,
$\frac{P}{Q}=\frac{625}{R}=\frac{R}{676}$
$R^{2}=\sqrt{422500}$
$R=650 \Omega$