The correct choice is knock-on. It means there is an added effect. Something leads to something else.

I've never seen knock-on in my life. Maybe it's a UK thing?

As far as i know, knock-on means that one situation causes other situations to happen. In other words, chain of events...

In this sentence, they're using these words to illustrate how writers of dystopian stories will either remove elements of society (knock-off) to create the stories' dystopian settings or add on new elements to create a dystopian society (knock-on). But I wouldn't stress about knock-on vs knock-off since I have literally never seen knock-on used in my life and I believe in this case it's used only to highlight the meaning of knock-off. I hope this helps! Let me know if you have any questions :)