LOOK at the meaning of the marked OPTION:cacophony- a harsh discordant mixture of soundsLet's look at the meanings of the other given options: monotony- lack of variety and interest; tedious repetition and routineharmony- the quality of forming a pleasing and consistent wholesymphony- an elaborate musical composition for full orchestra, typically in four movements, at LEAST one of which is TRADITIONALLY in SONATA formHence, from the given meanings, we find that cacophony is the correct one-word substitute.