If you picked this answer choice, you might have interpreted it as referring to the flaw we identified in the stimulus. It's true that the argument makes a flawed assumption related to living conditions, since it assumes that the overcrowded living conditions that affect parts of Oldtown also apply to Spoonville, when Spoonville could have a very different population distribution and have no overcrowding.

But that is an assumption that living conditions that exist in one city must also exist in a similar city, and cause similar problems. If we just focus on living conditions that happen to exist in one particular city, and their relationship to health problems in that city, the argument actually does the opposite of what this answer choice says. The stimulus explicitly states that the health problems in Oldtown are caused by the crowded living conditions that exist in Oldtown. So the argument never assumes that health problems in a city cannot be caused by living conditions in that city, as the answer choice states.

If you picked this answer choice, you might have noticed that the word "widespread" is ambiguous. In theory, it could refer either to health problems that affect a high percentage of the population or that are spread over a wide geographical area. But notice that taking either interpretation, the flaw in reaching that conclusion would still be the same. Either way, the argument would still assume that the population is "overcrowded" in both Oldtown and Spoonville, when that doesn't have to be true.

In other words, though the term "widespread" is ambiguous, the more fundamental flaw in the argument is its assumption that since the total population and total area of the two towns are the same, the population must be distributed the same way over that total area. This isn't the same thing as somehow confusing the two concepts of total population and total area.

Irrelevant. We don't need to know what the specific health problems are that are mentioned in the stimulus, let alone whether lower average life expectancy is one of them.

Irrelevant. We don't need to know whether these health problems are easily treatable or not. We're just interested in whether the same health problems caused by overcrowding in Oldtown will also be prevalent in Spoonville.

This is correct. Don't be thrown off by the fact that this answer choice talks about the cities having "identical" population density. This is about overall population density — population divided by area — and it's true that Oldtown and Spoonville have the same overall population density, since they have the same overall population and area.

The flaw we identified is that the argument assumes having the same overall population density will mean a similar distribution of that density: i.e., if Oldtown is overcrowded, or has overcrowded sections, Spoonville will too. This answer choice points out that that doesn't have to be true. Just because the overall density is the same doesn't mean the population is distributed the same way. Oldtown could have some significantly overcrowded sections, whereas Spoonville could have a much more evenly distributed population with no overcrowding. and thus with better living conditions.