This is the 110th most frequent Polish word.
którą
"Którą" means "which" or "whom" (feminine, accusative or instrumental case) in English.
Relative pronoun, referring to 'book' in accusative.
To jest książka, którą kupiłem wczoraj.
This is the book that I bought yesterday.
Interrogative pronoun, questioning 'road' in accusative.
Nie wiem, którą drogę wybrać.
I don't know which road to choose.
Interrogative pronoun, questioning 'offer' in instrumental.