During the Civil War, Frederick Douglass served as an adviser to what US President?

Frederick Douglass served as an adviser to President Abraham Lincoln during the Civil War, making this the correct choice. Douglass, an escaped slave and prominent abolitionist, was a powerful advocate for the rights of African Americans and played a significant role in the fight for emancipation. He often communicated with Lincoln and urged him to take a stronger stance on ending slavery and to allow African Americans to serve in the Union Army.

Through Douglass’s efforts, he was able to influence public policy and advocate for the inclusion of Black soldiers in the Union forces, which ultimately added to the strength of the Union Army and showcased the commitment of African Americans to the cause of their freedom. His relationship with Lincoln was critical during this transformative period in American history.

The other options do not fit this context as they either represent individuals from different time periods, such as Washington and Jefferson, or were not directly involved in the Civil War advisory capacity like Ulysses S. Grant, who although a crucial General during the War and later President, did not have a direct advisory relationship with Douglass.