The conclusion of the passage is that "their (mainstream economic theorists) motive in advancing this theory must be something other than disinterested concern for scientific truth."
Why? Well, we know that mainstream economic theory holds that manufacturers, in deciding what kinds of products to manufacture and what form those products should have, merely respond to the needs and desires of consumers. But, most major manufacturers manipulate and actually create consumer demand.
Thus, because even mainstream economic theorists watch television, their motive in pushing this theory must be something other than disinterested concern for scientific truth.
So the claim that manufacturers manipulate and create consumer demand is merely one of the claims that supports the conclusion; since most major manufacturers actually manipulate and create consumer demand, the theorist's theory that the manufacturers are merely responding to consumer need no longer necessarily follows. Thus, since even these theorists are watching television, they must be motivated by something other than the mere concern for scientific truth.
So, as you can see, the claim sets up the conclusion, i.e. the conclusion is based in the claim--answer choice (A).
Hope that helps! Please let us know if you have any other questions.
