Why do you use “choisis” in this sentence?

Question

Quels bonbons tu as choisis? This is passé composé, right?  Isn’t “ tu as choisi” right?

Maarten Kamp · Accepted Answer

Juan, 
Well spotted ! 
This is an ‘inverted’ question structure which has the direct object ‘ quel bonbons ‘ preceding the compound verb conjugated with avoir. 
A special case, that has past participle agreement with the direct object - in this case masculine plural, giving rise to ‘ as choisis ‘. The grammar is covered at an advanced level B2, as per the link below. 
 https://progress.lawlessfrench.com/revision/grammar/special-cases-when-the-past-participle-agrees-in-number-and-gender-when-used-with-avoir-in-le-passe-compose-conversational-past