The correct answer is (a) 31 March.
Earth Hour is an annual event organized by the World Wildlife Fund (WWF). It is held on the last Saturday of March, at 8:30 pm to 9:30 pm local time. The event encourages people to turn off their lights for one hour to raise awareness about climate change.
In 2015, Earth Hour was observed on 31 March. More than 178 countries and territories participated in the event, with an estimated 1.8 billion people taking part.
The following are the options for the question:
(a) 31 March: This is the correct answer. Earth Hour was observed on 31 March 2015.
(b) 28 March: This is incorrect. Earth Hour was not observed on 28 March 2015.
(c) 27 March: This is incorrect. Earth Hour was not observed on 27 March 2015.
(d) 19 March: This is incorrect. Earth Hour was not observed on 19 March 2015.
I hope this helps! Let me know if you have any other questions.