This answer choice would weaken the argument. If supply and demand balance out long-term, then a potential issue to global ecological problems could be waiting until human demand balances out with the supply of Earth’s resources.

Negating this answer choice tells us it is impossible to determine the limitations of Earth’s supply, but this doesn’t make the limit disappear. Demand could still outpace supply, but we just won’t know what the exact limit of the demand is.

In order for all solutions to require reducing current demand, this answer choice must be true. If current demand does not exceed Earth’s sustainable supply, then there is no need to reduce human demand—simply increase the supply up to Earth’s sustainable limit.

The conclusion doesn’t state that it must be possible to fix Earth’s environmental problems. It only tells us that any potential solution requires reducing demand, regardless of whether or not this is feasible.

We already know Earth has a hard limit on its supply. The argument still works regardless of whether or not human demand further decreases that limit.