You are correct to have picked B.  It is the best answer among the choices given here. 

First, when taking multiple choice tests, you should always see if you can eliminate “all of the above” when that is given as a possible answer.  Often, as is the case here, some of the other answers contradict one another and so D cannot be correct.  Here, there is no way that A and C can both be right because you cannot both attack and protect a government.

A is not the right answer.  There were no actual combat actions taken to protect civil rights in the Civil Rights Era.  C is also not the right answer.  If the military had protected Southern state governments from national interference, it would not have helped promote civil rights since the Southern governments were opposed to civil rights. 

B is the best answer.  The military was integrated long before Southern society was integrated.