Рюкзак sounds exactly like german "Rucksack"
in ukrainian and russian it is a loanword from german
There are many German loanwords in Ukrainian ^^ Sometimes coming through Polish, sometimes directly from German.
Other examples: дах, пляшка, картопля, шприц, штраф, коробка (at least cognate with the German "Korb", not sure if a loanword), родзинка
I wish these "pick the correct word" tasks had a translation of what is correct and why you got it wrong
Is ось only for "here is "?
You use ось when you can point to the object with your hand, it's right here. Or when giving something to someone. It's usually translated as "here it is", "here you are" or maybe "over here" (@Vinnfred)