This leads to a different conclusion than the author's, so can't bridge the gap. We can see this by reformulating (A) into its contrapositive: uniform speed limits should not apply to non-high-speed roadways. Now it's more clear that this doesn't match the argument—the author never claims that a uniform limit shouldn’t apply to low-speed roads, just that it should apply to certain highways.

The author is trying to argue for uniform application of a particular speed limit on the basis of saftey, not merely on the basis of uniformity. The uniformity is important only insofar as it has the potential to increase safety. Notice (B) doesn't address safety. According to (B), as long as the speed limit is uniform, i.e., the same everywhere, then it's fine. But uniformity alone isn't enough. Our author specifically wants the speed limit to be at 75 mph. (B) doesn't deliver that. (B) is agnostic about the actual speed limit.

Another way to see the deficiency in (B) is to realize that the author isn't arguing for an existing measure to be applied nation-wide, but rather for a new national measure of 75 mph to be implemented.

This describes a necessary condition for a national speed limit, but that would only be useful in determining that certain roadways should not have a uniform speed limit. Since the author claims that there should be a uniform national speed limit, this principle doesn’t help; it leads to the wrong conclusion.

This both has the wrong trigger and leads to the wrong conclusion. The author doesn't discuss any long-standing laws (or violation of laws), meaning the facts given don't trigger this principle. Further, the author's conclusion doesn't involve any judgment about what is or isn’t a good law, so this principle can't bridge to the correct conclusion.

This is the bridge we're looking for. The premises tell us that uniform speed limits (set at 75 mph on level, straight highway stretches) reduce accident rates, so this principle is triggered. Based on the principle, such a measure should be implemented, which matches the author’s conclusion that a uniform limit should be set nationwide. This is a more general bridge than we strictly need, but it does correctly bridge the gap.