It was thought they might rebel. The war was over their freedom, after all. The purpose of the war was actually to let the black people gain their freedom. Having a thought that they might rebel is just naive and irrelevant.
The blacks were denied the chance to be in combat during the early part of Civil War because it was thought that they might rebel. The correct option among all the options that are given in the question is the third option or the last option. I hope that the answer has come to your help.
