Accidents involving drivers who do not wear seatbelts have been shown

OFFICIAL EXPLANATION

Driving without a seatbelt is expensive for taxpayers, as these drivers often get seriously injured. We can strengthen the evidence for this argument by making the “often” or “serious” parts of the evidence more explicit. In this case, (E) works as a strengthener, as the frequency (“often”) of accidents on such roads (“particularly dangerous”) is notable.

Providing information about another problem with drivers on the road doesn’t do anything to highlight the frequency or seriousness of injuries caused by drivers not wearing seatbelts (A). We want to strengthen the seatbelt argument, not create an entirely new one.

If most citizens already wear seatbelts (B), it would not be necessary to implement a law to get them to do so. It is not necessary to address a non-existent problem.

Similar to (A), (C) addresses a different problem from the issue of drivers failing to wear seatbelts. Even if these motorcycle accidents are more serious, they are not linked to this argument in a direct way.

Finally, if public health did not improve with this measure (D), that would provide evidence against, not in favor of, this argument; the passage is arguing that public health is affected enough by this issue that it should be addressed by law. If public health isn’t affected, there would be less need for the law.