There are a couple of issue with (C). First, it only refers to "some" telemarketers, "some" could be as few as one and as such, is not helpful in explaining the general trend. Second, (C) would be a more attractive answer choice if the conclusion said that number of telemarketers convicted of fraud has remained modest despite the number of incidents as (C) merely demonstrates that the victims typically detect the fraud too late to actually trace and catch the perpetrators. (C) cannot explain the number of complaints though, even if the fraud is detected later one, nothing in theory prevents a victim from filing a complaint with the law enforcement even though the chances of these complaints resulting in arrests are law.
(E) is correct because it directly addresses the conclusion - how can we explain a low number of complaints given a high number of incidents? (E) tells us that most people are too embarrassed to admit they've been defrauded, hence they prefer not to contact law enforcement or seek any other legal remedy.
Let me know if this makes sense and if you have any further questions.