The correct answer is D. loading of the transformers not in proportion to their kVA ratings.
When two transformers are connected in parallel, the current flowing through each transformer will be proportional to its impedance. If the transformers have unequal impedances, the current flowing through each transformer will also be unequal. This can result in one transformer being overloaded while the other transformer is underloaded.
Option A is incorrect because short-circuiting of the secondaries would require a much higher current than would be caused by unequal impedances.
Option B is incorrect because the power factor of a transformer is determined by its load, not its impedance.
Option C is incorrect because the core losses of a transformer are independent of its impedance.
I hope this helps!