Answer:
C. Thus, Earth Hour is a necessary step against climate change and every person must join in.
Explanation:
Option C is the main idea of the passage, so it would be best to conclude the passage with this statement.
Option A is incorrect because this is introduction to Earth Hour and not a conclusion.
Option B is incorrect because nowhere in the passage the author gives such a suggestion. He/she just suggests it should be regularly observed as it does not cause much inconvenience, however if it is observed daily or monthly, it may cause inconvenience. Reaching such a surprise conclusion would not be good, because the author has not presented any arguments in support of this suggestion.
Option D is incorrect because this is not precisely the topic of the passage. The topic is Earth Hour and not global warming or climate change.