The "Night and Fog" decree was considered successful because
C. People vanish without a trace, and no information was provided about their whereabouts or fate
Step-by-step explanation:
Night and Fog was issued by Adolf Hitler as a directive to his army and secret force personnel in 1941 to target political activists and whoever was helping the resistance against Nazi rule in Germany and its occupied territories at the time.
They were to be imprisoned or killed according to the level they were deemed dangerous for the regime and their family and the population in general were to remain in the dark when the deeds had happened.