In a balanced wheat stone bridge, current in the galvanometer is zero. It remains zero when

 (1) battery emf is decreased

 (2) all resistances are increased by 20 ohms

 (3) all resistances are made three times

 (4) the battery and the galvanometer are interchanged

Balanced condition $\frac{\mathrm{P}}{\mathrm{Q}}=\frac{\mathrm{R}}{\mathrm{S}}$

This condition is independent of emf of cell, hence if battery emf is decreased the bridge will remain balanced means no current will flow through the galvanometer. 

Also, if resistances are increased three times, we get

$\frac{3\mathrm{P}}{3\mathrm{Q}}=\frac{3\mathrm{R}}{3\mathrm{S}}$

$\Rightarrow \frac{\mathrm{P}}{\mathrm{Q}}=\frac{\mathrm{R}}{\mathrm{S}}$

Also, $\frac{\mathrm{P}+20}{\mathrm{Q}+20}\ne \frac{\mathrm{R}+20}{\mathrm{S}+20}$