Is this a homework question?, if so theres a section for that.
The flux density in the core of the tranny will increase as the frequency decreases, this is because at lower frequencies volatge across the windings is there for longer giving time for the density to increase(volt seconds).
It depends very largely on the transformer (some trannys wuill completely saturate accross the entire freq range you mentuioned) however, this is a very 'loose' answer, but might satisfy a college lectrurer.