The the Ku Klux Klan formed to -b. To protect the voting rights of Southerners.
What was the reason?
The Ku Klux Klan was a white supremacist terrorist group that emerged during Reconstruction. It took violent steps to undermine the Republican party, hoping to maintain black economic instability and ensure white racial and economic superiority in the postwar South.
Congress countered the KKK with the Force Acts and the Ku Klux Klan Act of 1871, which made Klan violence and political intimidation illegal under federal law.
Republicans lost hold within the South largely due to Klan violence, allowing the South to maintain a ruling racial order that morphed into Jim Crow.
Hence, option b. is correct.