Impressionist music used a technique that was the opposite of harmony and that used disagreeable sounds. this technique was known as
melody
atonality
dissonance
syncopation

DISSONANCE is the technique used that is the opposite of harmony and combines disagreeable sounds.

Dissonance in music is the combination of tones that are not in accord with each other. It gives out an inharmonious sounds giving the impression of being incomplete and in a state of unrest.

Its the 3rd one i think