What was the outcome of the Battle of Antietam?

B) Lee's northward advance was stopped.

Explanation:

The battle of Antietam was the bloodiest battle in the American Civil War, and resulted in a pyrrhic victory for the Union, in which Lee's Northward advance was stopped, resulting in him retreating to recuperate his losses. This is also one of the major steps for the Union to go on the offensive, and would continue to be so with only a small delay in the battle of Gettysburg.