The correct answer is: "The size and scale of the camps"
The Nazi authorities used death camps in order to put into practice the massive extermination of Jewish people, which they called the Final Solution to the Jewish Question. When the death camps were liberated, their explicit purpouse was revealed, as many were disguised as labor camps, and even the German population believed so.
But the most shocking feature was its size and the industrial manner in which they could murder human beings at such a large scale, by using the gas chambers and crematoriums. Prisoner camps had been used in armed conflicts before, but never this format of death camps. It was believed that 5-6 millions of Jews were extermined by the nazi regime, but nowadays experts infer even larger figures.