The louder the sound, the greater the:

The correct answer is related to the concept of sound waves. Amplitude refers to the height of the sound wave, which is directly associated with the perception of loudness. When the amplitude of a sound wave increases, the sound becomes louder; conversely, a lower amplitude results in a softer sound.

Therefore, as you hear louder sounds, what you're actually experiencing is an increase in amplitude. Frequency, on the other hand, relates to the pitch of the sound and is measured in hertz. Timbre refers to the quality or color of the sound, which distinguishes different types of sound production, such as instruments or voices, even if they have the same pitch and loudness. Pitch is related to frequency but does not directly correlate with the loudness of a sound. Hence, the loudness of sound being associated with greater amplitude is the key reason why this choice is the correct answer.