I too lean towards E, but, I believe D is the best answer.
(D) 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.
the argument is that the trend will continue in future, but what if the reason that led to this trend are not effective in leading this in future? This is like a Sine curve, it on it's peak now, it may reaches the bottom in the future.
(E) It uses after-tax income as the base for computing the national savings rate without establishing by argument that after-tax income is a more appropriate base than before-tax income.
appropriate or not, after-tax income provides the indication.