it could have been a driver that was not actually driving the bus
A driver stopped the bus/
it could have been a driver of another car or bus that caused that bus to stop.
A driver stopped the bus
is a possible translation
and should be reported
this was already reported
If we are already talking about THE bus, then it probably has one specific driver at the moment. So it wasn't stopped by a random driver, but by THE driver of that bus.
I don't agree with you.
It is possible that the bus has more than one driver and we do not know exactly which of them stopped the bus.
By the way, the answer "A driver has stopped the bus" is correct.
So why "A driver stopped the bus" is not?