Of these options, the best is C. A major goal of the KKK was to prevent African Americans from voting.
The KKK was organized mainly to destroy the power of the Republican Party in the South. Once they did this, they felt, they would be able to reinstitute a system of Democratic Party rule that would assure white supremacy and generally make things more like they were before the war. In order to do this, one thing the KKK needed to do was to prevent blacks (and white Republicans) from voting. They did this through violence and other forms of intimidation.