Answer: VERBAL IRONY
Explanation: Verbal irony is a contrast between what is said or written and what is really meant. When one's speeches or writings are different, contrary or are in contrast to what he or she truely meant, that is verbal irony; for example calling an ugly man handsome.
The word on the wrought iron gate which reads ARBEIT MACHT FREI, translates to WORK SETS YOU FREE. This is a verbal irony because what is written is contrary to what happens behind the gate.
Non of them were ever freed due to their hard work but they were rather gassed to death when they are sick and unable to work.