Maybe more than one person owns the same belt (two sisters have one belt they both wear). Or two people made one belt together (say, two designers). It's a strange sentence but not entirely impossible given the right context.
I know, it didn´t make sense to me either, so I wrote their belts are white... and go it wrong. Was it supposed to trick us, or just confuse us??? They need to start making up sentences that make sense!
I agree. If the sentence was about belts nobody would have a problem with this. If its to show "delaS" can gi with a singular noun it is easy to think if a sentence that makes sense. Their house is white.
Their blet is white = Seu cinto é branco (formal) ; O cinto deles é branco (if it is a group of entities of unknown gender or if there is at least one entity of masculine gender) ; O cinto delas é branco (if all entities in the group have feminine gender)