A source emitting sound of frequency 680 Hz moves at 20 m s^-1 directly towards a large flat wall. A stationary observer stands behind the source on…

The source moves away from the observer, so the direct sound is Doppler shifted down: $f_1=680\times\dfrac{340}{340+20}=642\ \mathrm{Hz}$, hence (A) is false. The wall is a stationary receiver ahead of the source, so it receives $f_2=680\times\dfrac{340}{340-20}=722.5\ \mathrm{Hz}$, and it re-emits the same frequency; the observer, being at rest, hears $722\ \mathrm{Hz}$, so (B) is true. The beat frequency is $f_2-f_1=722.5-642.2=80.3\ \mathrm{Hz}$, so (C) is false. The wavelength in the medium ahead of the source is $(v-v_s)/f=320/680=0.471\ \mathrm{m}$, not $0.50\ \mathrm{m}$, so (D) is false. Only (B) is correct.