But the question states "Most populous European country not using the Euro", not "Most populous Euro member country not using the Euro". So UK is a correct answer. Actually to the quiz maker, if you take it by the letter of the law the answer should include Turkey (use Lira, twice as populous as Poland).
Minor issue: "Most populous European country not using the Euro as its official currency" should be Rank 4 (not Rank 3), because there are 3 countries ahead of it.
https://worldpopulationreview.com/country-rankings/total-fertility-rate
https://notesfrompoland.com/2021/04/08/baby-doom-poland-records-one-of-eus-lowest-fertility-rates/