If you really want to confuse an “environmentalist”, ask them “What is the optimum environment?”
I think that's a silly question. You might as well ask a doctor "What is optimum health?". The question is only confusing because nobody knows for sure the right answer and the right answer varies by person and by climate. But everyone knows and can distinguish between a poor state of health and a good one. Same with the environment.
Better to stick with logical arguments than trick questions.
That’s easy. One without humans. The odd meteor or two will tear up the planet every once in a while.