"Who likes slow busses?"
Translation:Кому нравятся медленные автобусы?
The dictionary entry says it all: it's BUSES now that we're in the 21st century. I always thought the duolingo team were young & sprightly but evidently they like to roll out archaic expressions sometimes!
Because the verb нравятся must agree with the grammatical subject of the sentence, which is the plural noun phrase медленные автобусы