Population standard deviation vs sample standard deviation

Hi, PFA I have a question here: Why is that you use population standard deviation formula? because we just have list, which means we have to use the sample standard deviation formula that is divide by n-1. There is this mix up in some places, where the answers use the population formulas (that is divide by n) even though we are given a sample. Then the answer for this question is option a, where n =11.

See When to use Population STD vs Sample STD if not specified? - #3 by Leaderboard