Here are the answers for each sentence based on the comma rules:
- Mom flew to Chicago and landed at O’Hare International Airport, one of the busiest airports in the U. S. Appositive
- If I win tickets to the concert, I will go with my best friend. Long Introductory Element
- We visited many of Rome’s landmarks, but we didn’t get into the Colosseum. Compound Sentence
- Leo ran down the block, across the street, through the park, and into the library. Items in a Series