Chlorine in the stratosphere is believed to come primarily from cfcs rather than from natural sources such as volcanoes because:

Chlorine in the stratosphere is believed to come primarily from CFCs rather than from natural sources such as volcanoes because the rise in the concentration of chlorine present in the stratosphere coincides with the rise in the concentration of CFCs in the same period of time. Also, there are studies which measured samples of air from volcanoes which shows that the volcanoes have a very small contribution in the amount of chlorine found in the stratosphere. 

The correct answer is  b)