To answer this question we need to remember a bit of the earth's history. In the first stages, Precambrian, on earth, there was no oxygen, nonetheless with the rise of photosynthesis by cyanobacteria the concentration increased, nonetheless it was a gradual process, now also there was an increase in carbon dioxide levels at the end of the Mesozoic due to intense volcanic activity along with other factors. So taking this into consideration we can say that the correct answer is the third option, as a volcano is a sudden event and if large enough maybe it won't change all the atmosphere, but can generate a local change.