Correct Answer - B
As no relative motion is there between observer and listener, hence frequency heard by observer is 512 Hz. He will observe frequency reflected from wall, `f_1=515Hz`. Hence, the wave reflected from wall will act as another source of frequency 515 Hz.
Therefore, the frequency received by the observer from wall is
`f_2=(v+v_0)/(v)f_1`
`=(342)/(340)xx515=518Hz`
Hence, beats observed is `f_2f=518-512=6`.