Translation:Ils arrivent !
What's the difference in pronunciation of 'il arrive' and 'ils arrivent'?
As far as i know, there is no difference. You have to rely on the context of the sentence to know which phrase is being used. However in this case, there is no context so both should be accepted.
Sorry i meant to say that if you got this sentence as a listening exercise then the answers "il arrive" and "ils arrivent" should be accepted.