The purpose of President Obama in this section of teh speech is to inspire people to collectively work hard toward a better future. Option C is correct.
In this speech, Obama contrasts the tough past they endured facing difficulties they had to resolved by proving themselves resilient; to the present which shows a slightly recovery which will advance if all Americans join their efforts and work together for the improvement of their country.
"My fellow Americans, we are made for this moment, and we will seize it -- so long as we seize it together."