Q.The walls of the hall built for music concerts should (A) amplify sound (B) reflect sound (C) transmit sound (D) absorb sound

[!TLDR] Good auditorium acoustics needs echo and reverberation minimised, which is achieved specifically by making the walls/surfaces absorpt …

Section 8.7.3 lists the conditions for good acoustics in an auditorium: sound must be heard loudly enough everywhere (achieved via a parabolic surface specifically BEHIND the speaker/performer, at their focus -- not via the general walls of the hall), while echoes and reverberation must be ELIMINATED or reduced, explicitly stated to be achieved 'by making the reflecting surfaces more absorptive'. If the general walls of a music hall simply reflected sound freely (option B) or transmitted it outward (option C, which would also let outside noise leak in and violate the sound-proofing requirement), the hall would suffer exactly the reverberation and echo problems the chapter warns against; 'amplify so …

Recall the auditorium-design conditions from section 8.7.3, distinguishing the SPECIFIC parabolic reflecting surface behind the speaker (for loudness) from the GENERAL wall …

Confusing the one specific parabolic reflecting surface recommended behind the speaker with a blanket instruction for ALL the hall's walls to reflect sound -- the general walls should absorb …