Answer:
A. The sinking of the Maine.
Step-by-step explanation:
The USS Maine was the battleship of the United States that sank in the Havana Harbour on February 15, 1898. The ship sank due to an explosion whose origin is still unknown. Many crew members of the ship died in the explosion.
This event instigated the United States to declare war over Spain, ultimately leading to Spanish-American war in April, 1898.
Therefore, the correct answer is option A, the sinking of the USS Maine was the event that ignited Spanish-American War.