Angry southerners joined groups like the red shirts because a they blamed the north for ending slavery. b they wanted to meddle in northern affairs. c they needed an organization to rebuild their cities. d they wanted to extend slavery into the north.

They joined the red shirt because (A). they blamed the North for ending slavery. 
The red shirts was a famous white supremacist group that came into power during the last years of the reconstruction period.
They believed that by Ending the slavery, the north has damaged the economy of southern states which rely on slaves for their plantations.

The theory is land mass basically what they said.