Final answer:
The sentence indicating President Arthur's presence at the Brooklyn Bridge opening best supports its national importance.
Step-by-step explanation:
The short story "Behold the Brooklyn Bridge" appears to be a piece of historical fiction highlighting the significance of the Brooklyn Bridge's opening. Among the provided sentences, the one that best indicates the national importance of the event is:
"Papa glanced over toward the crowd. 'Why that's President Arthur himself!' he exclaimed."
This sentence suggests the presence of a prominent national figure, which implies that the event was substantial enough to attract the attention of the United States President. This attendance reflects the event's importance to the nation, as presidential appearances are typically reserved for events of significant historic or cultural value.