d
Let \(q =\) charge on each drop

\(V =\frac{ Kq }{ r } \text {-.- (1) }\)

Now for combination of 64 drop

\(64 \times \frac{4}{3} \pi r^3=\frac{4}{3} \pi R^3\)

\(R=4 r\)

And \(Q =64 q\)

Potential of bigger drop

\(=\frac{ KQ }{ R }=\frac{ K 64 q }{4 r }=16 \frac{ Kq }{ r }\)

\(=16 \times 10\,mV =160\,mV\)

Correct answer is \(160 .\)