Survey of Subscribers: 30% of all orders placed by them were by people under 35 (Rest 70% people were 35 or above).
Survey of Advertisers: Most of the orders placed were by people under 35.

What must be true:

(A) More subscribers to Systems who have never ordered merchandise in response to advertisements in the magazine are age thirty-five or over than are under age thirty-five.
Beyond the scope of the argument since it talks about "never ordered". We have information only on last year orders.

(B) Among subscribers to Systems, the proportion who are under age thirty-five was considerably lower last year than it is now.
Beyond the scope of the argument since it talks about "now". We have information only on last year orders.

(C) Most merchandise orders placed in response to advertisements in Systems last year were placed by Systems subscribers over age thirty-five.
Not true. Most of the orders placed were by people under 35.

(D) Last year, the average dollar amount of merchandise orders placed was less for subscribers under age thirty-five than for those age thirty-five or over.
Not known. Dollar amount is not discussed. We only know about the number of orders.

(E) Last year many people who placed orders for merchandise in response to advertisements in Systems were not subscribers to the magazine.
True. Most people who had ordered were below 35 years of age. And only 30% of the subscribers who had ordered were under 35. So who were the others who were under 35 but had placed orders? They were non-subscribers. Hence this is correct.

Answer: E