Final answer:
Option A is correct because the Emancipation Proclamation signaled to the world that slavery would not be tolerated in the future United States, leading to the Thirteenth and Fifteenth Amendments that formally abolished slavery and granted Black men the right to vote.
Step-by-step explanation:
The Emancipation Proclamation, while a significant historical document, did not immediately free all slaves; it only applied to those in rebellious states and was not enforceable in areas without a Union military presence. Option A is the most accurate because it reflects the broader impact of the Proclamation, signifying a clear message to the world about the future intolerance of slavery in the United States. This can be understood with the context that the Thirteenth Amendment would later officially abolish slavery, and the Fifteenth Amendment would grant voting rights to Black men. Although the Emancipation Proclamation did not immediately free all slaves, it was a critical step towards these later constitutional amendments and was part of a broader change in attitudes towards slavery following the Civil War.