Here's how it works:
* Choisir is the infinitive form of the verb, meaning "to choose."
* The past participle is used in compound tenses, like the passé composé, which is formed with the auxiliary verb "avoir" + past participle.
For example:
* J'ai choisi un gâteau. (I chose a cake.)
* Ils ont choisi la meilleure option. (They chose the best option.)