Correct Answer - B
The frequency heard directly from source is given by
`f_1=((v)/(v-v_S))f`
Here `v=340(m)/(s)`,`v_S=2(m)/(s)`,`f=512Hz`
`f_1=(340)/(338)xx512=515Hz`
the frequency of the wave reflected from wall will be same (no relative motion between wall and listener, so no change in frequency). Hence no beats are observed.