This is a Strengthen question, and so let's dive into the argument. A government study tells us that raising the speed limit to actual average speeds on a kind of roadway reduces the accident rate. Then, we're told that this average speeds is 75 mph. The author then concludes that 75 mph should be the uniform national speed limit for level, straight stretches of all such roadways. The author here implicitly assumes that we SHOULD do something that reduces the accident rate, as he jumps from an empirical fact to a normative assertion. We need something that helps us make this bridge between "is" and "ought," and (E) makes this implicit assumption explicit.
(B) is wrong because the stimulus refers to "level, straight stretches of all such roadways" and so we don't need to consider other kinds of roads, which weren't considered in the government study. Hope this helps!