Final answer:
Harriet Tubman contributed to the Union war effort most directly by leading a Union raid that freed over 700 enslaved people. This raid, among her other efforts, highlights her crucial role in the fight against slavery and the Civil War.
Step-by-step explanation:
Harriet Tubman's contributions to the Union war effort are vast and significant. The correct answer to the provided question is Letter C: Harriet Tubman led a Union raid that resulted in over 700 enslaved people being freed. This raid, known as the Combahee River Raid, occurred in June 1863 and was particularly notable for Tubman's role as the first woman to lead an armed military operation in American history. Although Tubman did help enlist Black soldiers, the raid is her most direct contribution to the Union effort and her involvement in freeing enslaved individuals exemplifies her commitment to abolition and equality.
During the Civil War, African American troops, including freed slaves and Northern free Black people, made up a considerable portion of the Union forces. Their bravery in battles was instrumental in challenging and changing the views that questioned their effectiveness and value in combat. Ultimately, they played a crucial role in the Union's victory.