Which proclamation freed slaves held in states that were considered to be in rebellion?

The Emancipation Proclamation is the significant historical document that declared the freedom of all slaves in states that were in rebellion against the Union during the American Civil War. Issued by President Abraham Lincoln on January 1, 1863, the proclamation aimed to weaken the Confederacy by undermining its labor force, as enslaved people were crucial to the South's economy and war effort.

By declaring that all slaves in the rebel states "shall be then, thenceforward, and forever free," it not only transformed the character of the Civil War by making the fight for freedom a central goal but also set the stage for the eventual abolition of slavery throughout the entire nation with the passage of the 13th Amendment.

The other options mentioned do not correspond to any historical document or legislation related to the emancipation of slaves. The Freedom Declaration, Slave Release Act, and Rebellious States Act are not recognized or significant in the context of American history concerning the end of slavery. Therefore, the Emancipation Proclamation is the accurate choice, reflecting its importance and impact during a critical period in U.S. history.