Is it me or does it really sounds more like "Corpores ascenditis" instead of "Arbores ascenditis"?
"Arbores" can be either nominative or accusative plural. The correct answer assumes it is accusative and thus the direct object. In poetic languages, it might we used in the nominative case: You trees climb or with additional words, "You trees are climbing to the sky." I agree the correct answer is more likely, but if this text were from Ovid, the alternative translation might not be unexpected.