Article: In 2010, Indira Gandhi International Airport (IGIA) was conferred the fourth best airport award in the world in the 15–25 million category, and Best Improved Airport in the Asia-Pacific Region by Airports Council International. The airport was rated as the Best airport in the world in the 25–40 million passengers category in 2015, by Airports Council International.[not in citation given][better source needed] Delhi Airport also bags two awards for The Best Airport in Central Asia/India and Best Airport Staff in Central Asia/India at the Skytrax World Airport Awards 2015.

Question: What year did Indira Gandhi International Airport win an award for Best airport in the world in the 25-40 million passenger category?
2015