Order to reduce the loudness of a sound, we have to
$(a)$. decrease its frequency of vibration of the sound
$(b)$. increase its frequency of vibration of the sound
$(c)$. decrease its amplitude of vibration of the sound
$(d)$. increase its amplitude of vibration of the sound

The loudness of a sound gets increased by increasing its amplitude and gets decreases by decreasing its amplitude.

So, option $(c)$ is correct.