B, let residents of residential buildings wear anti-noise earmuffs to reduce noise, but it is not suitable for the actual situation, so B is wrong;
C, noise monitoring device installed in residential buildings, can't weaken the noise, so the c error is big;
D. The noise is mainly due to the loudness of the sound it makes, so asking the aunts to lower the tone of their voices in the square dance can't weaken the noise, so D is wrong.
So choose a.