What is the principal reason for incorporating a city? a) for the convenience of the residents who live there b) to work in partnership with the county government c) to formally enlarge a town that preexisted d) to increase the power of a state government

a) For the convenience of the residents who live there.

Let's consider why that answer is correct -- what convenience or value residents gain by having an incorporated town or city.  An unincorporated community is really just a traditional view of a certain area of residents. There are no elected officials and pursuing much change to the infrastructure of the area is difficult.  There really isn't a clear way to purse local democracy in an area where people have settled but where there isn't an incorporated town or city for governing purposes.

So maybe we'd say that the value of incorporation amounts to even more than simply "convenience" of the residents who live there.  It's part of the process of pursuing planned improvements and infrastructure and participatory government to the area.