b
Source is stationary
$ \Rightarrow \lambda = {\rm{constant \,\,and\,\, f'}} = \frac{{{\rm{v}} + {{\rm{v}}_{\rm{s}}}}}{{\rm{v}}}$
$\mathrm{f}=\left(1+\frac{\mathrm{v}_{\mathrm{s}}}{\mathrm{v}}\right) \mathrm{f}=\left(1+\frac{1}{5}\right) \mathrm{f}=1.2 \mathrm{f}$