Explanation:

- Original Sentence: "I bought five dozen onions today." This sentence is clear and grammatically correct.

- Option 1: "dozens of onion" - Incorrect. "Onion" should be plural, as it refers to multiple items.

- Option 2: "dozens of onions" - Incorrect in the given context. We generally use "dozen" without an "s" when referring to a specific quantity.

- Option 3: "dozens onions" - Incorrect. "Dozens" should be followed by "of" if used at all, but it's not appropriate here.

- Option 4: No improvement - The original sentence is correct. No improvements are needed here.