Well I would have made the same mistake you made if I hadn't noticed from the question that we are basing on data from a sample and not from a population. If you were really using your knowledge from what you learnt on this platform and not from "numpy" you would have easily nailed this question because from the lessons they did not teach us about calculating variance of population based data and so they questioned what they taught. But nevertheless I think they should stressed out that point in the tutorials so that we wouldn't be having these issues.