Which circumstance does not require attribution?

a journalist reporting on the scene what is happening at that time

a journalist interviewing an expert source who wishes anonymity

a journalist quoting a politician's remarks at a dinner in her honor

a journalist quoting a member of the public at an accident scene

The circumstance that does not require attribution is A. a journalist reporting on the scene what is happening at that time.

Attribution means giving credit to the original author. So, all other options here demand the journalist to give them credit for their contribution to his story, whereas an event that is currently happening (and where he is not using anyone's personal materials) does not.