Why is Pearl Harbor significant?

It was an important naval base.

The U. S. declared war on Japan after the attack on Pearl Harbor.

It was the center for sending troops to fight in Europe.

B. The U. S. declared war on Japan after the attack on Pearl Harbor. While it was an important naval base, and that was the reason that the Japanese attacked it, in the grand scheme of things it didn't matter all that much, considering the U. S. at the time was focusing its naval efforts on the Atlantic rather than the Pacific, though it did have one eye open. However, when the Japanese attacked Pearl Harbor, that's when the Pacific got hot and war was declared.

tl; dr the answer is B