Can a 50Hz transformer be used for a 25Hz if the input voltage is maintained constant at the rated value corresponding to 50Hz?

a)      Yes. Since the voltage is constant, current levels will not change.

b)      No. Flux will be doubled which will drive the core excessive saturation.

c)       No. Owing to decreased reactance of transformer, input current will be doubled at load.

d)      Yes. At constant voltage, insulation will not be overstressed