The Mississippi River which is over 2300 miles long is the second-longest river in the United States.
1. The Mississippi River which is over 2300 miles long is the second-longest river in the United States. (No commas)
2. The Mississippi River, which is over 2300 miles long is the second-longest river in the United States.
3. The Mississippi River which is over 2300 miles long, is the second-longest river in the United States.
4. The Mississippi River, which is over 2300 miles long, is the second-longest river in the United States.

4). The Mississippi River, which is over 2300 miles long, is the second-longest river in the United States.

Explanation:

As per the question, option 4 i.e. 'The Mississippi River, which is over 2300 miles long, is the second-longest river in the United States' most appropriately revises the given sentence to rectify the punctuation error. It correctly employs comma(,) before and the after 'participial phrase' ('which is over 2300 miles long') that involves non-essential description and removing this would not affect the basic meaning of the sentence. Therefore, the comma separates or offsets this non-essential description from the sentence. Thus, option 4 is the correct answer.

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