I mentioned in a caveat that the country can be a colony and still abolish slavery within its borders. Because the colonies mentioned exist now as independent countries and have not re-introduced slavery, the first mention of slavery abolishment is valid. If I created the quiz with colonies not included, it would cause a country that abolished slavery centuries earlier to instead have an abolishment date at a more recent independence date (which sounds unfair to me).
Should Russia be given credit as a world-leader in the abolition of slavery when serfdom persisted there until 1861? Wikipedia: "As with slaves, serfs could be bought, sold, or traded (with some limitations as they generally could be sold only together with land, with the exception of the kholops in Russia and villeins in gross in England who could be traded like regular slaves), abused with no rights over their own bodies, could not leave the land they were bound to, and could marry only with their lord's permission."
I am not well acquainted with Russian history, I am only going off this statement that is associated with the date above: "Peter the Great converts all house slaves into house serfs, effectively making slavery illegal in Russia." If you think that is invalid, you can propose to me a new date and provide a link with a source. This list was difficult to come up with because there are many loopholes to abolishment of slavery.