Step 1: Understanding the word "mellifluous."
"Mellifluous" means something that sounds sweet or pleasant, especially a voice or soun(D) Its opposite should describe something unpleasant, harsh, or grating in soun(D)
Step 2: Analyzing the options.
- Resonant refers to something rich in sound, often deep or clear, but it does not indicate unpleasantness.
- Cacophonous means a harsh, jarring sound, which is the exact opposite of mellifluous.
- Harmonious refers to sounds that are pleasing and in agreement, which is similar to mellifluous.
- Acrimonious refers to something bitter or hostile, usually used to describe language or behavior, not sounds.
Step 3: Conclusion.
"Cacophonous" is the correct antonym, as it describes something harsh and unpleasant in soun(D)
Thus, the correct answer is 2. Cacophonous.
Final Answer:
\[
\boxed{\text{The correct answer is 2. Cacophonous.}}
\]