Official explanation from VeritasAs you assess your Decision Points here, looking at the underlined portion and differences between answer choices, you should recognize the familiar symptoms of a list that needs to be made parallel. In such a situation, the GMAT will often begin the list before the answer choices, so pay strict attention to the fixed information before you start assessing parallelism. Here you're stuck with the first item (of three): "the city's deep pool of health enthusiasts."
Note the possessive "the city's," which could carry across to all three items in the list (three items all belonging to the city) or which may necessitate that each item have its own article (e.g. "a," "the," etc.) or possessive (e.g. "its" or "the region's").
In choice A, "retail market" - the second item - does not have an article, so it belongs under the umbrella of "the city's." But the third item introduces another possessive, "its," and therefore breaks parallelism.
Choice D commits the same error replacing "its" on the third item with "the city's" Since items one and two each fall under the original "the city's," this extra article breaks the parallel structure of the list - either all three each get their own possessive, or only the first does and that possessive controls the whole list.
Choice E essentially makes the same mistake, replacing the possessive "its" in C and "the city's" in D with "a" - an article that breaks parallelism because the second doesn't have anything preceding it while the first and third do.
And choice C has a similar error: while "a retail market" and "a growing community" are parallel, the extra preposition "of" breaks parallelism. Since "avail themselves of" already has an "of" that applies to the first and second terms, adding it to the third term breaks that structure.
Only
choice B preserves the parallel structure, giving each item its own possessive or article ("the city's deep pool...a retail market...and a growing community"), and allowing all three to sit under the umbrella of the lone "of" in "avail themselves of."
Choice B is therefore correct.