Answer:
Option b. Antietam
Step-by-step explanation:
The first major Union victory of the Civil War was not at Bull Run, which was actually a Confederate victory. The bloodiest battle of the Civil War was also not at Bull Run, but rather at Gettysburg. The battle of Antietam was the bloodiest single-day battle of the Civil War and was considered a Union victory. Chancellorsville was a Confederate victory, and Fort Sumter was the site of the first shots of the Civil War, but it was not a major battle and did not result in a Union victory. So, the answer to your question is Antietam.