Answer : 3. "18th"
Explanation :

1. The 18th G20 summit has concluded in New Delhi.

2. The G20 was established in 1999 after the Asian financial crisis as a forum for finance ministers and central bank governors to discuss global economic and financial issues.

3. In the wake of the 2007 global economic and financial crisis, the G20 was elevated to the level of heads of state/government, and in 2009 was designated as the "major forum for international economic cooperation".