Which Union Army officer commanded the Army of Northern Virginia?

The Army of Northern Virginia was actually commanded by Robert E. Lee, who was a Confederate general and not a Union Army officer. He was known for his leadership during several key battles in the American Civil War and stood as a central figure in the Confederacy's military efforts. His strategic prowess was evident in battles such as the Seven Days Battles, the Second Battle of Bull Run, and the Battle of Gettysburg, where he solidified his reputation as one of the most skilled military leaders of the time.

The other options refer to Union commanders who were influential during the war but had no command over the Army of Northern Virginia. Ambrose Burnside and George McClellan were Union generals, while Ulysses S. Grant was the overall commander of the Union forces toward the end of the war. Therefore, understanding that the Army of Northern Virginia is directly associated with Robert E. Lee clarifies the answer to this question.