Answer:
The sentence from the story that best supports the idea that the event was of national importance is Papa glanced over toward the crowd. "Why that’s President Arthur himself!" he exclaimed.
Step-by-step explanation:
"Behold the Brooklyn Bridge" is a story that has a lot of elements and historical references around, but the line that makes clear that the event was of national importance shows that the character is surprised by seeing that the President is in the place for the event, this gives it the level of national importance since it was part of the President's agenda.