I can't seem to remember some of the English terms for chess pieces. The rook is a tower where I'm from and my dictionary says castle is a dated term for that piece. I think it should work.
The Nobel Prize for Economics is an add-on and is known officially as the Swedish Central Bank Prize in Economic Sciences in Memory of Afred Nobel. And as such is not accepted by many as a Nobel Prize inclulding the Nobel family;
But it's only considering short and informal names for countries. If it were asking fo their full official names then neither North or South Korea would count.
Oganesson is a seventh member of the Noble gases group 18. Discovered / confirmed in Dec 2015. Maybe include a comment to exclude it from the group of 6 better known elements?
I see that you aren't adding castle to rook even though the OED says it's an alternate name and the Wikipedia entry notes it as an informal name. Please reconsider.
There is a competition called World Marathon Majors, that include the six mentioned. QM didn't made that up, so it's not up to him nor either of us to change anything about the question.
https://en.wikipedia.org/wiki/Nobel_Memorial_Prize_in_Economic_Sciences#Controversies_and_criticisms
https://en.wikipedia.org/wiki/Oganesson
The six countries listed were original members of the European Coal and Steel Community, a forerunner to the EU.