c
In the absence of electric field \((i.e. E = 0)\)

\(mg = 6\pi \eta rv\)          …\((i)\)

In the presence of Electric field

\(mg + QE = 6\pi \eta r(2v)\)          …\((ii)\)             

When Electric field to reduced to \(\frac{{E}}{{2}}\)

\(mg + Q\,\left( {E/2} \right) = 6\pi \eta r(v')\)   \((iii)\)

After solving \((i)\) ,\((ii)\) and \((iii)\)

We get \(v' = \frac{3}{2}v\)