If you add the suffix ET to the adjective extreme, meaning exceeding usual limits what part of the speech results in what is the new meaning

This would now be an adverb because it’s explaining the action verb. If you were to say “she has taken exercising to a new extremity,” the extremity is explaining the manner the action verb is in, making the most sense. The noun is “she,” the verb is “exercising” so the only option left really is for it to be an adverb.