The question is asking us to answer Which sentence uses commas correctly?. We see here that Savannah is the name of the city and that the city is already mentioned - the name is an extra information. You could for example say "The southern city is a beautiful place". This means that the information about the city name is added, and an added, unnecessary information is surrounded by comas on both sides - so the correct answer is
c. The southern city, Savannah, is a beautiful place.
