This is an evaluate question. We need to identify the central assumption and find an answer choice that, when answered, would either strengthen or weaken the conclusion.

The argument is: Because 92% of motorcyclists involved in bad motorcycle accidents have never taken a safety course (evidence), there would be fewer bad motorcycle accidents if more motorcyclists took safety courses (conclusion).

A shift in scope underpins the central assumption: that motorcyclists involved in bad motorcycle accidents (the sample) are representative of motorcyclists in general (the group). We have data on the sample (92% don't have training), but we need a way to compare that data to similar data for the group as a whole.

So we can expect that the answer will address the sample's representativeness in some way.

A - Correct. If significantly more than 8% of motorcyclists in general (the group) have taken a safety course, we can then compare that to the implied 8% of the sample who have taken a safety course. For instance, if 25% of the group has taken a safety course, but only 8% of the sample has taken a safety course, then there's a strong negative correlation between motorcyclists who have taken safety courses and those who have been involved in bad accidents. This strengthens the argument. Now let's say that only 1% of the group has taken a safety course; that means that there's a strong positive correlation between motorcyclists who have taken safety courses and those who have been involved in bad accidents. This weakens the argument.
B - Out of scope; argument says nothing about passengers
C - Also out of scope; we're not interested in comparing between different safety courses.
D - A few previous posters selected this, so let's puzzle it out. It mentions the 92% proportion and so might address representativeness--but it only mentions the 92% in connection with collisions with other vehicles in motion, which is irrelevant. Note that we're no longer talking about the 92% of motorcyclists involved in bad accidents without training; we're talking about 92% of all bad accidents. This is a shift in scope.
E - Also out of scope; we're not interested in the influence of size/speed, but taking vs. not-taking a safety course