Only "one". Some of those attacks count as basic attacks, so they are the default attacks for Opportunity Attacks. Some "single" attacks are powers that let the monster attack twice or more with one basic attack.
So the young black dragon, with its standard action, could attack once with its bite, or once with its claws, or twice with its claws with its "Double Attack" power.
If its breath weapon was a minor action, it could attack with it, too, but it isn't.
The black dragon also gets to make some extra attacks when its opponents meet certain conditions.