The correct response to the effect of the Emancipation Proclamation on the Union's ability to fight the Civil War is: It allowed the Union Army to recruit African American soldiers.
Although that option wasn't listed fully in your message, the Emancipation Proclamation primarily allowed for the recruitment of formerly enslaved people into the Union Army, which significantly bolstered the Union's manpower and morale. It also shifted the war aims of the Union to include the abolition of slavery, which helped to undermine the Confederacy's labor force and increased support for the Union cause, particularly among abolitionists and in Europe.