Final answer:
Lincoln's 'great task remaining before us' was to dedicate themselves further to the objective of concluding the Civil War, with the primary goal being the reunification of the country.
Step-by-step explanation:
According to President Abraham Lincoln, the 'great task remaining before us' was to increase their devotion to the cause of finishing the war. Lincoln's primary goal immediately following the Civil War was reunifying the country (C), as he believed in the nation's obligation to the slaves and in the importance of the process of reconciliation. His commitment to the abolition of slavery was steadfast, and he saw the Thirteenth Amendment as a way to ensure the permanent end of slavery, a cause for which many had sacrificed greatly.
While Lincoln did support the idea of extending the right to vote to black men after the war, his main focus during the conflict was to keep the Union together and to abolish slavery. This reflects the broader themes of Lincoln's presidency and his views on what would constitute a successful resolution to the Civil War.