The correct answer is A: Many free African Americans volunteered to fight for the Union.
The Emancipation Proclamation, issued by President Abraham Lincoln in 1863, declared the freedom of enslaved people in Confederate states. While it did not immediately free all enslaved individuals, it did encourage free African Americans and former slaves to enlist in the Union Army, significantly boosting their numbers. The other options are not accurate representations of the effects of the Emancipation Proclamation.