I think its D because he didnt know that blacks were being discriminated that bad.
You use the semicolon with an independent clause when you join two of the independent clauses in a sentence without using a conjunction. So basically the semicolon replaces the conjunction when you're combining two independent clauses.
Occasion: Divisive times during the American Civil War over the issue of slavery
Audience: The American public/population
Subject: The issue of slavery
Speaker: Abraham Lincoln
Purpose: To unite the American population