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."
This is because the sentence includes items in a list (the cities) that each contain a comma (the city name and state), which can lead to confusion. Using semicolons would clarify the separation of the items in the list:
"This summer, we are going on a road trip to Philadelphia, Pennsylvania; Baltimore, Maryland; and Rehoboth Beach, Delaware."