b
The ground state velocity of the electron in the hydrogen atom

is, \(v_{0}=2.18 \times 10^{6} m / s\)

The expression for the speed of the electron in fourth orbit of \(H\) atom is,

\(v_{4 th }=v_{0} \frac{z}{n}\)

\(=2.18 \times 10^{6} \times \frac{1}{4}\)

\(=0.545 \times 10^{6} m / s\)

\(=5.45 \times 10^{5} m / s\)

Thus, the speed of the electron in fourth orbit of \(H\) -atom is \(5.45 \times 10^{5} m / s\)