What would cause oil to leak into the coolant system on 1995 Volkswagen Jetta

I have flushed my jetta twice and there still oil in the coolant system. With that I have done thermostat water pump and a few hoses..many people re saying its either the freeze plugs, head gasket, or valve cover gasket

Asked by for the 1995 Volkswagen Jetta
What kind of oil is mixing with the coolant,Is it engine oil than u have to check the engine oil cooler and if its transmission oil u have to check the radiator.