My two cents' worth here:
1. An increase in the proportion of people who reported for PTSD treatment does NOT ALWAYS mean that a greater NUMBER of people actually suffer PTSD, which has (according to the passage) resulted from more chaotic times or people being more sensitive to violence.
2. It could mean the same number of people who have always suffered PTSD, but that more people have now come forward to say that they're down with PTSD. That means it is not necessarily the case that these are more chaotic times nor that people are more sensitive to violence, which the argument is structured to imply an increase in the number of people reporting for PTSD (but then again, refer to point 1 - increase in proportion does not mean an increase in the total number of people suffering from PTSD).
3. Since we need to look for a defending assumption i.e. protect the conclusion from being destroyed or attacked fatally, we have to assume that either people are NOT more likely to come forth for PTSD treatment, which means the increased proportion is actually due to more people getting sensitive to violence or that these are indeed chaotic times.
Hence C is the answer.
1. An increase in the proportion of people who reported for PTSD treatment does NOT ALWAYS mean that a greater NUMBER of people actually suffer PTSD, which has (according to the passage) resulted from more chaotic times or people being more sensitive to violence.
2. It could mean the same number of people who have always suffered PTSD, but that more people have now come forward to say that they're down with PTSD. That means it is not necessarily the case that these are more chaotic times nor that people are more sensitive to violence, which the argument is structured to imply an increase in the number of people reporting for PTSD (but then again, refer to point 1 - increase in proportion does not mean an increase in the total number of people suffering from PTSD).
3. Since we need to look for a defending assumption i.e. protect the conclusion from being destroyed or attacked fatally, we have to assume that either people are NOT more likely to come forth for PTSD treatment, which means the increased proportion is actually due to more people getting sensitive to violence or that these are indeed chaotic times.
Hence C is the answer.