D. loading of the transformers not in proportion to their kVA ratings.

A. Short-circuiting of the secondaries: This is incorrect. Short-circuiting of the secondaries would occur if the primary and secondary windings of the transformer were connected together, but this is not the case with two transformers connected in parallel.

B. Power factor of one of the transformers is leading while that of the other is lagging: This is also incorrect. Power factor refers to the ratio of real power to apparent power in an AC circuit, and is not affected by the impedance of the transformers.

C. Transformers having higher copper losses will have negligible core losses: This is also incorrect. Copper losses refer to the energy lost due to the resistance of the transformer’s windings, while core losses refer to the energy lost in the transformer’s core. Both types of losses will still occur regardless of the impedance of the transformers.

D. Loading of the transformers not in proportion to their kVA ratings: This is correct. When two transformers with different percentage impedances are connected in parallel, the one with the lower impedance will carry more of the load, leading to an unequal distribution of power between the two transformers. This means that the transformer with the lower impedance will be loaded more than its kVA rating while the transformer with the higher impedance will be loaded less than its kVA rating.