Hello, kim,
This question is one of the more abstract types of Reading Comprehension questions, and thus can be a little trickier to deal with at first than others. But it's okay, practice makes these easier same as any other question type.
The assumption that the question refers to Lowe making is described in the last paragraph, lines 57-62, and also on lines 9-11, that when no new monuments are built, then the site had been abandoned at that time. This neglects the possibility that, for example, Mayan sites only built monuments for a little while until they had all necessary monuments, then stopped building monuments after that. There are any number of explanations for why monuments had stopped being built at a given site other than "because the site was abandoned at that time".
That said, answer choice E is best, because it compares the assumption that the friend has sold her stamp collection (the assumption that a site has been abandoned) because the friend has stopped purchasing new stamps (because no new monuments have been built) to Lowe's assumption, in the manner just described.
Hope that helps,
Lucas Moreau
