c
(c) Electric current corresponds to the revolution of electron is \(i = \frac{{ev}}{{2\pi r}}\)
Magnetic field due to circular current at the centre \(B = \frac{{{\mu _0}}}{{4\pi }}.\frac{{2\pi \,i}}{r} = \frac{{{\mu _0}}}{{4\pi }}.\frac{{ev}}{{{r^2}}}\)

==> \(r = \sqrt {\frac{{{\mu _0}}}{{4\pi }}.\frac{{ev}}{B}} \)==> \(r \propto \sqrt {\frac{v}{B}} \).