To select the appropriate option that completes the diagram, we need to understand the context of the diagram. If the diagram is related to significant events or documents regarding slavery and emancipation in the United States, then the option that best completes the diagram would likely be related to the abolition of slavery.
Given that context, "The Emancipation Proclamation" would be the most fitting choice, as it was a key executive order issued by President Lincoln that declared the freedom of slaves in Confederate states.
Therefore, the answer is:
The Emancipation Proclamation