Answer: The answer is C: U.S. citizens must dedicate themselves to the preservation of the nation and freedom for all.
Step-by-step explanation:
While the answer may seem like B due to the third paragraph's focus on the soldiers you know the answer is C from the lines "it is for us the living, rather, to be dedicated here to the unfinished work which they who fought here have thus so nobly advanced" and "...that this nation, under God, shall have a new birth of freedom-and that the government of the people, by the people, for the people, shall not perish from the earth."
With these two lines together you can see that Lincoln, who we know issued the Emancipation Proclamation, wants everyone to A) keep fighting for freedom and B) keep defending their system of government.