School integration plans that involve busing between suburban and central city areas have contributed, according to a recent study, to
significant increases in housing integration, which, in turn, reduces any future need for busing.
A highly debatable question. The reasoning is that social integration plans lead to increases in housing integration first and these increases (not the housing integration per se) cause the reduction in any future need for additional busing. (A) significant increases in housing integration, which, in turn, ----
'which' refers to housing integration and not to the increases. If it were to refer to the increases, then the verb should be the plural 'reduce' rather than 'reduces'. This is a logic error. (B) significant integration increases in housing, which, in turn, reduces -- 'which' should refer to the integration increases and not housing. The same problem as in A.
(C) increase housing integration significantly, which, in turn, reduces --
to increase is an infinitive and therefore 'which' refers just to housing integration but not the increases. (D) increase housing integration significantly, in turn reducing --
the adverbial modifier 'in turn reducing' modifies the previous clause school integration plans have contributed to -- This is the only choice that seems to have some semblance of rationale. (E) significantly increase housing integration, which, in turn, reduce --
Basic S-V error If one ignores the relevance of logic, one may go for A, as the 'which' apparently seems to refer to the 'integration'. In that case, B also could be a hot contender with the relative pronoun modifying 'housing'. All in all, this is a confusing question.