E is correct.
The argument states that the only reason why reading skills among high school students have been declining as the result of overcrowding in schools.
Option E implies that there are other reasons for decline in reading skills.
Option A is not relevant to the passage's argument.
Option B is tempting as it does state that overcrowding is not always associated with lower reading scores. But it does not addresses the passage argument on whether overcrowding is the ONLY cause of lower reading scores.
Option C is irrelevant to the argument.
Option D is similar to Option B, and is thus wrong.
Reading skills among high school students in Gotham have been steadily declining, which can only be the result of overcrowding in the schools.
Which of the following, if true, most seriously weakens the argument expressed above?
a) The high school system in Gotham succeeds in giving students a good education at considerably less cost than do most systems.
b) Several cities have found that overcrowding in the schools is not always associated with lower reading scores.
c) Gotham schools have a greater teacher-to-student ratio than most other school systems.
d) Students' reading skills have not declined in other cities where the high schools are just as crowded as those of Gotham.
e) Schools are not overcrowded in many cities where high school reading scores have declined more than they have in Gotham.