Question

how is noise pollution best described? an unpleasant scent or smell a chemical contaminant in the air airborne chemicals unwanted or disruptive sound

Explanation:

Noise pollution is related to sound. The first option is about scent/smell (not sound). The second and third options are about chemicals (not sound). The fourth option "unwanted or disruptive sound" correctly describes noise pollution as it pertains to sound which is unwanted or causes disruption.

Answer:

unwanted or disruptive sound