D - voyages and exploration
The correct answer is: "social media has become an important tool in presidential campaigns".
The graph shows the number of related Tweets (measured in millons) that were published online during the 2009 and the 2012 presidential campaigns in the US. The figure has grown from 10 million Tweets in 2009, to over 30 million in 2012. If using this figure as a benchmark to measure the influence of social media, it can be claimed that it has grown a 200%.
I think that what Mandela means when he speaks of nation building is creating a world where everyone is equal and has rights. I think this because we know that there was a huge racial injustice at the time and Mandela was fighting for black rights. He also used the word reconciliation which means making a view or belief compatible with another or the restoration of friendly relations.
I hope this helped.