Translation:Those who are young are standing on the bus.
The problem with the given English translation is that it is ambiguous. It can either mean ‘The young ones are standing (not sitting) in the bus’ or ‘It is the young ones (not the old ones) that are standing in the bus’. However, the Hungarian sentence can only mean the latter, since it constrasts the young with the old, and not standing with sitting. So a proper English translation would be: ‘It is the young ones that are standing in the bus’ or ‘Those standing in the bus are the young ones’. Actually, the Hungarian sentence is also a bit too artificial. It would be better said as “A FIATALOK állnak a buszon.” with the word FIATALOK being stressed, which indicates that this is the focus of the sentence, thus it contrasts with the old. (If the sentence was “A fiatalok ÁLLNAK a buszon”, with the work ÁLLNAK being stressed, it would mean that the word ÁLLNAK is in the focus and it would contrast with sitting.)
Those stand on the bus, who are young. - This is NOT a good sentence in English. Secondly those standing on the bus was wrong, but sounds more like American English - 3rd your hint says youths - we don't use youths - unless posessive then youth's - The English sentence would say something like Young people are standing on the bus. I am quite certain that the Hungarian sentence is not translated that way, but you are 1000 times more likely to hear someone speak what I wrote that what the "correct" answer is stated.