Final answer:
The Emancipation Proclamation freed slaves in Confederate States, not in Union States, and shifted the objective of the Civil War increasingly toward ending slavery.
Step-by-step explanation:
The Emancipation Proclamation
The statement that best describes the Emancipation Proclamation is that it declared that slaves in Confederate States are freed, but not slaves in Union States. This is accurately reflected in option a) of the multiple-choice question. When President Abraham Lincoln signed the Emancipation Proclamation on January 1, 1863, it stated that all persons held as slaves in states or regions in active rebellion against the United States were considered free. However, the proclamation did not apply to slave-holding border states that remained in the Union or to Confederate regions already under Union control. Thus, the proclamation had a limited immediate impact on slavery but became a significant step towards the eventual abolition of slavery in the United States.