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 <u>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</u>. Therefore, the comma separates or offsets this non-essential description from the sentence. Thus, <u>option 4</u> is the correct answer.