I initially got this wrong and marked C, but now I see why D is the right answer:
Let's focus on the arguement at hand:
A developer wants to build a shopping mall in a mostly rural area of a mid-sized city. In the letter to the city council requesting a building permit,
the developer argues that the permit should be granted because the shopping mall, if successful, will ultimately benefit the city by creating jobs in a place where the unemployment rate is slightly higher than the national average. Ok, intersting wording! Focus on the bold portion and especially look there is "if successful". We are talking about only 1 case! The malls are successful!
Coming to answer choices
A) Irrelevant to reasoning. This may demonstrate that local residents support the mall, but it does not necessarily aid the developer’s position before the city council.
B) Irrelevant to reasoning. City building codes are not relevant to the discussion of whether a new shopping mall will benefit the city financially.
C) Irrelevant (as we are only talking about when it is successful). This choice is a trap, because it indicates some kind of assurance that the mall will work, but that's something that should not be our focus here. Our real focus needs to be on strengthening the developer’s petition before the city council (i.e if malls are successful then these areas that are having high unemployment will benefit) Not whether the mall will be successful or not! I hope this helps as I saw a lot of confusion here on the forum

D)
Correct Oh this is interesting!
I will like to mark this as "the angel is in the details" in my error log!"A developer wants to
build a shopping mall in a mostly rural area of a mid-sized city." while in his argument the developer says "will ultimately benefit the city by creating jobs
in a place where the unemployment rate is slightly higher than the national average."
In a mid-sized city there are some prosperous places (where unemployment rate is less than the national average) and some not so prosperous places (where unemployment rate is higher than the national average)
what a mix! Then the developer's arguement has assumed that they are building in the less prosperous areas which will benefit if the mall is successful! Otherwise the arguement goes for a toss!
Think about the option "The city’s largest percentage of unemployed people lives in the rural area where the proposed shopping mall would be built." This tells us we are avoiding those prosperous areas! A Clear Srengthener!
E) Irrelevant to reasoning. This answer choice, though provides a good argument, falls outside of the scope of the original statement.
Hope this helps!