The sentence that requires semicolons instead of commas is:
"This summer, we are going on a road trip to Philadelphia, Pennsylvania, Baltimore, Maryland, and Rehoboth Beach, Delaware."
In this sentence, the locations include city names followed by state names, which creates a complex list. Therefore, using semicolons can help avoid confusion. The corrected sentence would look like:
"This summer, we are going on a road trip to Philadelphia, Pennsylvania; Baltimore, Maryland; and Rehoboth Beach, Delaware."