What bus should I take from Royal Park Hotel to the Hong Kong Airport if I have a flight at 7:50am?

You need to reserve about 2 hours at least for your flight and you'd better get to the airport at 05:50. And the travel time from the hotel to the airport by public transportation is about 60 - 120 minutes. And at that time, most buses do not start so you'd better take a taxi to make sure you won't miss your flight. And you can ask the hotel you live in to help you book one taxi in advance.