This strengthens the argument by addressing a potential weakener – if most people who drive long-distance only own one car, they're pretty likely to use that car the wrong way. Maybe they buy a light car and drive it on the freeway (that's unsafe). Maybe they buy a heavy car and drive it around town (that's inefficient).

(A) says "nono don't worry that doesn't happen." Most people who drive long-distance own two cars, so they can get a heavy one for the freeway and use the light one locally.

(B) says the number of cars on the freeway is higher now than it was 20 years ago.

Does this connect to fuel efficiency? In this respect (B) almost weakens, because it suggests there may be a lot more heavy (i.e. fuel inefficient) cars around. It doesn't actually weaken, though, because we've still established most traffic is local. Still, it sure as heck doesn't strengthen.

Does this connect to safety? If anything, more cars on the freeway seems less safe. More traffic might mean more accidents.

(C) does an excellent job strengthening the part of the conclusion that says we saved fuel, but it does so in a way that undermines the other part of the conclusion – that we preserved safety.

All the cars, even the big ones, are lighter now. Yay! Perhaps they're more fuel-efficient! By the same token, though, perhaps they're less safe on the freeway.

Commercial vehicles aren't mentioned in the stimulus at all. That's not bad on its own – plenty of correct answers in Strengthen questions bring in new concepts – but it's bad in this case because there's no clear connection between commercial vehicles and safety. Do they make highways less safe because they're so big? Do they make highways more safe because they tend to move slowly?

Manufacturers [did the "two types" strategy] 20 years ago. Our conclusion is about the effects of [that strategy]: it saved fuel, and it preserved safety.

(E) discusses stuff that happened before [the "two types" strategy]. A few manufacturers were messing around with this idea before it became widespread. The story of how the strategy came into being is irrelevant to the argument's discussion of its effects.