Option 1 -> Resonant means deep, clear, and echoing sound - not opposite to mellifluous.
Option 2 -> Cacophonous means harsh, discordant, and unpleasant sounding - direct opposite of sweet-sounding.
Option 3 -> Harmonious means melodious and pleasing - similar to mellifluous, not opposite.
Option 4 -> Acrimonious means bitter or angry in tone - refers to attitude rather than sound quality.
Hence, Option 2: Cacophonous -> MELLIFLUOUS means sweet-sounding and smooth, while CACOPHONOUS means harsh and jarring, making them exact antonyms in the context of sound quality -> correct