In fact, the "elongated rectangle" hint is not very helpful, as there is a large number of flag in the 1:2 formats and other, which make them look "elongated" - thus, this shape is not really that unusual.
Sri Lanka should be removed. A flag with a ratio of 1:2 is not at all uncommon. However, flags with ratios shorter than 2:3 are uncommon, as others have mentioned.
Btw love the quiz.