It is A. The subject of the sentence is “species” and the highlighted phrase needs to agree with it… In B, “the frog” is wrong because it is not necessarily referring only to the particular species. In C, “they” is wrong because it cannot refer to species which is one of the those collective singular nouns. D has the same problem as B besides having an ending phrase that is ackward. E has the same problem as C besides also having unacceptble phrases like “easy finding.”