After a series of losses early in the war, Union troops turned the tide by winning an important battle at__.

A. Bull Run
B. Fort Sumter
C. Richmond
D. Antietam

The correct answer is D. Antietam.

To arrive at this answer, we need to have some background knowledge about the American Civil War.

The American Civil War was fought between the Union, which consisted of Northern states, and the Confederacy, which consisted of Southern states that had seceded from the Union. The war began in 1861, and initially, the Union faced a series of losses.

Now, to determine which battle turned the tide for the Union, we can eliminate some options based on historical context:

A. Bull Run: The first Battle of Bull Run took place in July 1861, and it was one of the early battles of the American Civil War. Although it was a Confederate victory, it did not turn the tide of the war in favor of the Union.

B. Fort Sumter: Fort Sumter was the site of the first shots fired in the American Civil War when Confederate forces attacked the fort in April 1861. However, this event did not turn the tide of the war.

C. Richmond: Richmond was the capital of the Confederacy, and although there were several battles fought around Richmond during the war, the Union did not win an important battle there that turned the tide.

D. Antietam: The Battle of Antietam took place on September 17, 1862, near Sharpsburg, Maryland. It was the bloodiest single-day battle in American history, with significant casualties on both sides. Although it was a tactical draw, it was a strategic victory for the Union as it halted the Confederate advance into Northern territory. This battle is often seen as a turning point in the war because it dissuaded European countries from recognizing the Confederacy and gave President Abraham Lincoln the opportunity to issue the Emancipation Proclamation.

Therefore, based on these considerations, the correct answer is D. Antietam.