Which is the best version of the underlined portion of sentence 13 (reproduced below)?
Agile and athletic, C. rechenbergi rear up on their back legs and lash out when attacked—or it somersaults away, rolling like tumbleweed.

The plural “they” agrees with the plural noun “C. rechenbergi,” and “somersault” appropriately maintains the present tense used throughout the paragraph. (“C. rechenbergi” is consistently plural in number here, as signaled most closely by “rear” earlier in sentence 13.)
Choice A is incorrect because “it” does not agree with the plural noun “C. rechenbergi.” Choice B is incorrect because “it” does not agree with the plural noun “C. rechenbergi” and because “will somersault” inappropriately shifts the sentence into future tense. Choice D is incorrect because “were somersaulting” inappropriately shifts the sentence into past progressive tense.