During world war ii, the tide turned in favor of the united states at the battle of
a. leyte gulf.
b. midway.
c. okinawa.
d. the coral sea.

The correct answer is: it was the battle of Midway. 
The battle of Midway took place in the beginning of June 1942 and it was focused on the Midway Atoll, today uninhabited Atoll belonging to the US. 

The battle was decisive for two reasons: US retained the possession of the Atoll, which is very strategically placed, and the Japanese Navy received a great amount of damage, which they never managed to repair.