Thanks for your answers but I still do not understand something .. Which one of these oscillator settings should be chosen for a robust operation .. Or in other words , for a "classic proper" operation ... What I understand from your explanations is that I can choose any of these settings non regarding if the OSC is internal or external ...

As far as I know the Frc stands for internal oscillator ... Please correct me If I'm wrong at this point .. So If I choose 4MHZ with PLL operation , I'm running it with 32 MHZ internal oscillator which is also called the internal RC osc block , so in this case I should go with Frc for a more accurate conversion ... or with Fosc/32 or Fosc/2 for a faster conversion ... Right ?