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:
Well, not all the events throughout the country are going to have the presence of the President, then when in the story "Behold the Brooklyn Bridge" it is mentioned by the principal character with a huge surprise because the President actually went to the place for the event and he gave it a big importance to the situation, since it was part of the President's agenda.
Hope this helps.