Why did the nothern italy have a bigger role in the expansion of trade in europe

The most likely reason has been a better road connection (infrastructure has always played a big role in how important a certain region is and will be in the world). Another is the fact that the northern parts of Italy have always been more developed as Trieste, Milano, Venezia are all examples of cities in northern Italy that have had a major impact on our cultural, and scientific history in the past. With all that in mind, trade was also more easily conducted in cities that were more important and developed - where a major part was played by the same three countries. 

The correct answer is b

Yes thats true but what is the question