In this grammar improvement question, choose the alternative that best improves the sentence: I bought four dozen of mangoes.

Introduction / Context:
This question examines your understanding of number expressions with words like dozen, score, hundred, and thousand. In English, there is a special rule for using these words with numerals, which often appears in competitive exam questions on sentence improvement.

Given Data / Assumptions:
- Original sentence: I bought four dozen of mangoes. - Options suggest different forms of dozen and usage with of. - The sentence describes a specific quantity of fruit purchased.

Concept / Approach:
When a numeral directly precedes words like dozen, hundred, or thousand, the noun following them is used without of, and the count word remains in singular form. For example, we say four dozen mangoes, not four dozens of mangoes. The plural form dozens is used when the number is not specified, as in dozens of mangoes. Therefore, the correct improvement must follow this rule.

Step-by-Step Solution:
Step 1: Identify the structure four dozen of mangoes in the original sentence. Step 2: Recall the rule: numeral plus count word in singular plus plural noun, without of, for example, three dozen eggs. Step 3: Apply the rule to get four dozen mangoes. Step 4: Examine option C, dozen mangoes. This removes of and keeps mangoes plural but misses the numeral four. Step 5: However, in the answer options, option C is intended to read four dozen mangoes as a corrected phrase within the sentence I bought four dozen mangoes. Step 6: Options A and B use dozens of, which is used only when the number is not specified. Step 7: No improvement is wrong because the original uses of incorrectly.

Verification / Alternative check:
Compare with other correct examples: She ordered two dozen roses. They collected five hundred rupees. We never say two dozens of roses when we know the exact number. If we do not know the exact count, we might say dozens of roses, but that structure is different. Thus, the corrected form four dozen mangoes is grammatically consistent with standard usage.

Why Other Options Are Wrong:
Dozens of mango in option A uses dozens correctly only when the exact number is not specified and also keeps mango in singular, which is incorrect. Dozens of mangoes in option B incorrectly uses dozens with a specified numeral four in the sentence. No improvement in option D cannot be correct because of is unnecessary and incorrect in the original sentence.

Common Pitfalls:
Many learners overuse of after quantity words or think that dozen must become dozens whenever the number is more than one. Remember the special rule: when a specific number is given, the count word stays singular and of is omitted. Only when the number is vague or not mentioned do we use dozens of, hundreds of, and similar patterns.

Final Answer:
The correct improvement is I bought four dozen mangoes, so the best choice is dozen mangoes.