I'm assuming that this question is referring to the time after the Civil War called Reconstruction.
The Radicals in Congress wanted to closely keep an eye on the South and make sure that no one started another war or mistreated the slaves. The people of the South were still very bitter to the North and made several attempts to "make a comeback" however it all failed.
Have the military districts would prevent anything from happening again and protect the peaceful citizens of the South.
I believe that the answer is the second one To force Southerners into obeying federal Reconstruction.
