Which edition of "Earth Hour" was observed on 28 March 2015 across the world?

Which edition of  "Earth Hour" was observed on 28 March 2015 across the world?

Options:

1) 11th 

2) 14th 

3) 15th

4) 9th

Answer:

4) 9th 

Other Details:

• Over 170 countries participated in the event and more than 1,200 landmarks observed the Earth Hour
• Adopting a cleaner and more sustainable lifestyle by embracing renewable energy solutions is the aim of Earth Hour