The pronoun which one (lequel)

To ask "which one?" or "which ones?" in French, use the interrogative pronoun lequel. Because it replaces a noun, it must agree with that noun's gender and number.

SingularPlural
Masculinelequellesquels
Femininelaquellelesquelles
We want a package for the summer, but which one?
Nous voulons un forfait pour l'été, mais lequel ?
The salesman gave me these brochures, which one do you want?
Le vendeur m'a donné ces brochures, laquelle veux-tu ?
I want to do a cruise, which one is the least expensive?
Je veux faire une croisière, laquelle est la moins chère ?

Just add an -s to the singular forms to make them plural:

There are three excursions, which ones are included?
Il y a trois excursions, lesquelles sont incluses ?
Some meals have an extra charge, which ones?
Certains repas ont un supplément, lesquels ?

Asking "Which one of..."

To specify what you're choosing between, add de (of) immediately after the pronoun.

Which one of these cruises makes a layover in Rome?
Laquelle de ces croisières fait une escale à Rome ?
For the accommodation, which one is the most comfortable?
Pour l'hébergement, lequel est le plus confortable ?
The agency offers two package plans, which one do you prefer?
L'agence propose deux formules, laquelle préférez-vous ?