Answer:
George B McClellan
Step-by-step explanation:
Robert E Lee was a commander of the Confederate forces, not Union forces. This is why the first option is incorrect.
Jefferson Davis was the president of the Confederate States of America. This shows why the second option is incorrect.
Abraham Lincoln was president of the United States at this time and appointed George B McClellan as the commander of the Union army. This shows why the third option cannot be correct.