To increase the current sensitivity of a moving coil galvanometer...

New, current sensitivity, ${I}_s^{'} = {I}_{s} + \dfrac{50}{100}{I}_{s}$

$= \dfrac{150}{100}{I}_{s} = \dfrac{3}{2}{I}_{s}$

New resistance, $R^{'} = 2R$

Initial voltage sensitivity, ${V}_{s} = \dfrac{Is}{R}$

Now, new voltage sensitivity.

${V}_{s} = {i}_s^{'}R^{'} = \dfrac{\dfrac{3}{2}{I}_{c}}{2R} = \dfrac{3}{4}{V}_{s}$

$$ decrease in voltage sensitivity

$= \dfrac{{V}_{s} - {V}_s^{'}}{Vs} \times 100$

$= \left(1 - \dfrac{Vs}{Vs}\right) \times 100$

$= \left(1 - \dfrac{3}{4}\right) \times 100 = 25$