1.
Your Answer is
Correct Answer is B
Explanation
According to the meaning of the sentence "to completely clear the trees on the mountain to create new ski trails, although it is a one-time event, will destroy the fragile ecosystem and wildlife habitat" though leads to a concessional adverbial, as a parenthesis, So double commas are needed to separate them from the sentence stem.
Item A: There should be no comma after the mistake though.