"She should choose a major."
Translation:Zij zou een studie moeten kiezen.
I think wickeymuis meant that a major is only a part of your studies (next to the Dutch translation majoor it also means something like hoofdvak) so it's not a 1-on-1 translation to study/studie. If your study has a major/minor structure however I would guess your chosen study IS your major, and you fill it up with a minor, so if you still have to choose your major you still have to choose your study/studie? ;)