Answer:
D. All of these features make Philadelphia the best tourist attraction in the nation.
Step-by-step explanation:
The author of the passage is in general talking of the state of Philadelphia. He lists a lot of important monuments, places and famous historical references to the specific place. Also, he talks of the modern recently available accessibility of these famous places for any visitors.
But, these don't constitute the main viewpoint of the author. He is just using these facts and historical information to emphasize the importance of the state. His viewpoint is most evident instead in the sentence
All of these features make Philadelphia the best tourist attraction in the nation.