答案：B

解析：

The structure of the sentence indicates that the idea after the colon elaborates on or explains the first part of the sentence. It is possible that the members of the "rebel group . . . have never been punished" even though they have "committed," or carried out, numerous acts that break rules or laws. And the idea that the members have not been punished despite having committed violations does elaborate on the idea that the group has had "total impunity," or has been completely exempt from punishment and harm.