When you're tackling a strengthen or weaken question, you have to determine where there is a logical gap in the argument.

In this case, the flaw in the argument is that the author assumes that the pattern will repeat, regardless of store type: big store moves in -> some stores close as a result -> other stores move in to take their place

However, Colson's was a nondiscount store, and Spendless is a discount store. We would have to know what types of stores are closing and opening. If, when Colson's moved in, nondiscount stores closed, but discount stores opened in their place, then when Spendless moves in, the discount stores will close, but there may not be any other type of store that's able to replace them. Basically, we need to know whether stores are being replaced by the same type of stores, or by cheaper ones.

A. Many customers of Colson's are expected to do less shopping there than they did before the SpendLess store opened.
Our argument concerns the discount stores that closed because of Spendless, not Colson's. Irrelevant.

B. Increasingly, the stores that have opened in the central shopping district since Colson's opened have been discount stores.
If the stores that came in were discount stores, there might not be anything else that could move in to replace those. This weakens the idea that these locations will not stay vacant for long. Correct.

C. At present, the central shopping district has as many stores operating in it as it ever had.
Current number of stores tells us nothing about whether they will stay vacant in the future.

D. Over the course of the next five years, it is expected that Goreville's population will grow at a faster rate than it has for the past several decades.
This would strengthen the argument, if anything. More people would presumably lead to more stores and fewer vacancies.

E. Many stores in the central shopping district sell types of merchandise that are not available at either SpendLess or Colson's.
We don't care about current stores. We only care about whether new stores will replace the ones that shut down because of Spendless.

The correct answer is B.