Learn French: L'agence propose deux formules, laquelle préférez-vous ? - The agency offers two package plans, which one do you prefer?

Laquelle translates to "which one" and must agree in gender and number with the noun it refers back to. Here, it stands in for one of the formules (package plans). Because une formule is feminine singular, we use the feminine singular form laquelle.

Does formule just mean a mathematical formula?

Not always! While it can mean a scientific formula, in business and everyday French, une formule is widely used to mean an "option," "package," or "plan." You will see this word all the time for restaurant menus, gym memberships, and travel packages.

Why can't I say laquelle formule préférez-vous?

Because laquelle is a pronoun meaning "which one," so it completely replaces the noun. If you want to keep the noun in the sentence and say "which plan," you must use the interrogative adjective quelle instead, as in: quelle formule préférez-vous ?

Could I put laquelle at the end of the sentence instead?

Yes, in casual spoken French you will often hear people avoid inversion by saying vous préférez laquelle ? Putting the question word at the front and using subject-verb inversion (laquelle préférez-vous ?) is a bit more formal, which perfectly matches a professional setting like an agency.