This is the 240th most frequent German word.
welche
"Welche" means "which" in English.
'Welches' is an interrogative pronoun used to inquire about a specific noun, in this case, the book being read.
Welches Buch liest du gerade?
Which book are you reading currently?
'Welche' is used as an interrogative pronoun, asking about preferences among different options of colors.
Welche Farben magst du am liebsten?
Which colors do you like the most?
'Welche' is a relative pronoun, introducing the relative clause providing additional information about the documents.