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The right answer is B. The subject noun species is plural here (although the singular form is identical), and so the plural verb are is necessary. Furthermore, the correct idiom in English is are typical of; the phrase are usual of is simply not used.

The right answer is NOT F because wildflowers are not people, and so that/which would be used, not who. (It is acceptable to use either for animals, especially pets, so the test will not ask about that, only people vs. objects.)

The right answer is NOT G because which would introduce a non-essential clause and need to be preceded by a comma, and because those which are finding is grammatically incorrect in context regardless of whether there is a comma (the wildflowers are not doing the finding—people are finding the wildflowers).

The right answer is NOT H because the finding of the wildflowers is not hypothetical or conditional here—the sentence states that there are some in one kind of prairie and also some in another kind—and so if makes no sense in context.