Why does it seemingly, only sometimes allow the old spelling 'Kiev'. I underdstand its outdated but it's been accepable to use that spelling elsewhere in the course. I also agree that 'which' is a better word choice in English - though understand that is literally 'what' and gef that it's important to understand native sentence constructions.
Likely because with each exercise, there needs to be a report done on the spelling. There are only so few contributors and mods looking after Ukrainian right now. If some exercises are allowing/not allowing one spelling over the other, just report it and leave a comment about it. Some day, someone will see it and hopefully change it.