We are informed about the trend that some of the most prosperous nations in the world have experienced a pronounced drop in national savings rates.
The conclusion is: "this trend will undoubtedly continue if the average age of these nations' populations continues to rise,"
Why? Because "older people have fewer reasons to save than do younger people."
The first questions that should come to your mind is, just because older people have fewer reasons to save than younger people do, does that necessarily mean they won't save as much as young people? What if, even though older people have fewer reasons to save, they have more important and pressing reasons to save? Then the conclusion would no longer necessarily follow.
This is exactly what answer choice (D) is pointing out: "It only takes into account the comparative number of reasons older and younger people, respectively, have for saving, and not the comparative strength of those reasons."
Thus, answer choice (D) clearly indicates the error in the reasoning of the argument.
Hope that clears thing up! Please let us know if you have any other questions.
