The correct answer to this open question is the following.
Although there are no options attached we can say the following.
For the goal that states the need for a "more perfect union", to what were the Founding Fathers referring?
Answer:
The founding fathers knew that the Articles of Confederation(the first form of the Constitution) felt short to cover all the expectations of a true government. Indeed, under the Articles of Confederation, the central government was weak.
That is why they considered that the states needed a more érfect Union.
So, delegates of different states met at the Constitutional Convention in the city of Philadelphia, Pennsylvania, to decide on the new form of government for the hew nation and create the new Constitution.