Answer should be 'B'.

Students which were not doing good joined charter school and their performance went doen more because of chaning school.
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Vipin Gupta

I pick B.

Since most of the students at charter schools were performing badly at schools (as given by this statement), it is clear that the researcher's study is supported.

As far as D is concerned, its not necessary that the students in the charter schools were preciously studying in some other school. Most of these students can be new also. So this choice doesnt strengthen the researcher's study.
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Let's get it right!!!!

I would go with D. B seems to provide a different explanation...

D is the OA.

The OE is:
The conclusion that the act of changing schools is responsible for lower student performance relies on the assumption that the badly-performing students had in fact moved from a public school to a charter school. This is never made explicit, although the researchers’ conclusion depends on this being the case. The fact that most of the charter schools studied just opened would bolster this assumption, since students at these new charter schools must have enrolled in them recently. Choice (D) thus strengthens the argument by reinforcing its main assumption. Choice (A) is incorrect because the distinction between urban and rural students has no discernible impact on the distinction between public and charter schools. Choice (B) is incorrect because it does not in any way address the effect of changing schools, which is at the heart of the argument. Choice (C), if anything, weakens the researchers’ explanation by suggesting that at least in some cases, changing schools improved performance. As for choice (E), fundraising efforts are at least one step removed from the issue of student performance, although one could even argue that choice (E) weakens the argument by providing an alternative explanation for charter schools’ relatively poor performance—less money for teachers and materials.