How to calculate voltage regulation of three phase transformer correct in given case?
Take a look at the first sentence of the question.


  The bank is made by 3 identical transformers 7967/480V


When you connect the primary in wye configuration, then the primary line to line voltage is able to work with voltage until 7967V*root(3)=13.7kV. 

What question doesn't say is the bus voltage. In that case, if you suppose that the bus voltage is 7.9kV then you are right. If you suppose that the bus voltage is 13.9kV, then the  book is right. Thinking power grid, I'd say that 13.9kV is a line to line voltage to expect.