In addition to official restrictions, white Southerners blocked African Americans from voting by offering to end segregation. using violence and threats. secretly changing election dates. bribing them to stay away from polls.

I think it's B

using violence and threats

Explanation:

There were several official restriction, like for example in order to be able to vote you had to had been paying taxes for a certain amount of time, and many of the african american population were slaves so they couldn't pay taxes, for those african american that were free and were tax contributors and had all requierements to vote in line, white southerners used violence and threats to keep them from voting.