Step 1: Define "mellifluous."
"Mellifluous" denotes a sweet or pleasing sound, particularly a voice. An antonym should describe an unpleasant, harsh, or grating sound.
Step 2: Evaluate provided terms.
- Resonant: A rich, deep, or clear sound; not necessarily unpleasant.
- Cacophonous: A harsh, discordant sound, the direct opposite of mellifluous.
- Harmonious: Pleasing and concordant sounds, similar to mellifluous.
- Acrimonious: Bitter or hostile, typically applied to language or behavior, not sound.
Step 3: Determine the antonym. "Cacophonous" accurately describes a harsh and unpleasant sound, making it the correct antonym for mellifluous.Therefore, the correct option is 2. Cacophonous.
Final Answer: \[\boxed{\text{The correct answer is 2. Cacophonous.}}\]