Answer: (A) polarities of all transformers must be same.
Percentage impedance of all transformers operated in parallel should be same but if there is a difference, parallel operation is still possible but load sharing by the transformers may not be as desired. If voltage ratio of all transformers operated in parallel is not same, still parallel operation of transformers is possible but there will be local circulating current in-between transformers. We can always put different MVA rated transformers in parallel but load sharing would be according to there MVA ratings if other conditions are properly fulfilled. But if polarities two transformers are not properly connected there will not be any output at all since total current will locally circulated in between transformers no current will come to the load. We can compare this phenomenon with two oppositely connected battery.