"The ones who preferred the red box are now happy."
Translation:Kırmızı kutuyu tercih edenler şimdi mutlu.
'kırmızı kutuyu' is the direct object of 'tercih edenler' and therefore comes before the verb in this part of the sentence. As I see it you might rewrite the sentence to 'Kırmızı kutuyu tercih eden kişiler şimdi mutlu' - the subject noun of the sentence comes after the relative clause. With '... edenler ...' the subject is kind of integrated into the verb of the relative clause.