Answer
(a)
Work Step by Step
While atmospheric chlorine levels have declined significantly, full stabilization is a slow process due to the long atmospheric lifetimes of these compounds.
Stratospheric chlorine is decreasing due to the phasing out of ozone-depleting substances, and studies have projected its decline to levels below 3 ppb, which is a significant benchmark for stratospheric ozone layer recovery.
The answer is (a).
