$n' = \frac{{(c + v)}}{\lambda }$$ = \frac{{\nu (c + v)}}{c}$

Now these waves are reflected by the moving target

(Which now act as a source). Therefore apparent frequency of reflected second $n'' = \left( {\frac{c}{{c - v}}} \right)n'$ $ = \nu \left( {\frac{{c + v}}{{c - v}}} \right)$

The wavelength of reflected wave  $n = \left( {\frac{c}{{c - v}}} \right)n'$

The number of beats heard by stationary listener  $ = n'' - \nu  = \nu \left( {\frac{{c + v}}{{c - v}}} \right) - \nu  = \frac{{2\nu v}}{{(c - v)}}$

Hence option $(a)\, (b)$ and $(c)$ are correct.