Should be AC, the transformer equal to short circuit to DC voltage. With a current protected power source, you can try out what happen if you pump in the DC voltage. Hope the winding of the transformer dose not melt.
AC, as transformer is a AC device,(induced voltage v=do/dt, variation of flux with time variation, in square wave variation of time is not possible with time)would not work with DC. With DC reduced,inductive reactance would damage the winding. Refer my paper, " Magnetizing inrush current pattern for transformer......".
AC is the answer because in case of dc input supply no emf will be produced in primary winding. So net voltage in the primary winding will be (Vin-0) more due to which primary winding draws more current which leads to burning of winding
As high frequency transformer is used to make buck or boost or fllyback which changes the level of Dc( SMPS) so primary which we given in Pulsating DC.