"Quelle pomme avez-vous mangée ? La rouge ou la verte ?"
Translation:Which apple did you eat? The red one or the green one?
You're incorrect. Look up the rule "preceding direct object". When the object of the verb comes before the verb (as in "the apple I ate", "the men I saw") then the past participle must agree in number and gender.
la pomme que j'ai mangée
les hommes que j'ai vus
This also applies to pronouns, e.g. "je l'ai mangée" and "je les ai vus" for "I ate it" (if the "it" is feminine, as in apple) and "I saw them".