Spoiler alert: The answer is below the line.

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The right answer is A. Since the noun they precedes both the verb are and the verb contain, no comma is necessary after degraded. Furthermore, the information presented in this choice is the most relevant to the paragraph. (In this question, the right and wrong answers are determined by a combination of grammatical and stylistic factors—the test does not do this often, but it does happen, so watch out for it!)

The right answer is NOT B because this would result in two independent clauses joined by a conjunction without a comma in front of it (and, to a lesser extent, because the passage is about plantlife, not animal life, so information about birds is irrelevant).

The right answer is NOT c because this would result in a dependent “afterthought” clause that is not separated from the preceding independent clause with a comma (and, to a lesser extent, because the term flora refers to all plantlife, and so the phrase grasses and flowers and flora is redundant).

The right answer is NOT D because the passage is about diversity in prairie plantlife, not what interests tourists (and, to a lesser extent, because the idea that tourists from Western states would be more interested in degraded prairies than are tourists from other places is nonsensical).