C

Step 1: Understand the purpose of concert hall design.
Concert halls are designed to optimize the quality of sound for both performers and the audience. This involves how sound behaves within the hall.
Step 2: Define key terms related to sound behavior:
- Amplify sound: This means increasing the volume of sound.
- Transmit sound: This refers to allowing sound to pass through materials.
- Reflect sound: This involves bouncing sound waves off surfaces, enhancing their propagation within the hall.
- Absorb sound: This reduces the amount of sound that is reflected back, possibly leading to a dull sound experience.
Step 3: Identify the desired acoustic qualities of a good concert hall:
A successful concert hall must effectively reflect sound to ensure it reaches the audience clearly and powerfully. Reflection is crucial as it helps in distributing sound evenly throughout the space.
Conclusion: Therefore, the walls of halls built for music concerts should reflect sound to enhance acoustical quality.
Thus, the correct answer is C.