Sentence D does not make sense, as irony is when the opposite of what is expected to happen happens. You certainly wouldn't know what was in a mysterious box. Sentence C wouldn't work either, as it just isn't irony. Sentence A is not really irony either, so therefore sentence B makes the most sense. The reason being is because the military makes weapons that usually cause harm, so it is ironic that they are making a weapon that isn't violent but instead ensures peace.
