What about C?

Answer choice (C) is, in this context, too broad. The purpose of the lines in question is not to "identify the specific source of problems in enacting constitutional reforms in Canada" - that would apply to ALL constitutional reforms in the entire country. Answer choice (B), by contrast and in this context, is appropriately tailored - the purpose of these lines is "to locate THE source of A systemic problem in protecting ABORIGINAL RIGHTS in Canada." Hope this helps!

Systemic seemed a bit strong..we don't really know anything about the system as a whole, only as it relates to this issue? So how can we infer it is a systemic problem protecting all aboriginal rights?