This could be an alternative cause of the persistent fatalities. If speed limits were raised just as the seatbelt laws went into effect, then faster driving could have caused more fatalities, effectively cancelling out the reduction brought about by the laws.

This could resolve the discrepancy because it would mean that more kinds of deaths are now counted as traffic fatalities, making the current number of recorded fatalities inflated even if fewer people are actually dying.

This could be an alternative cause of the persistent fatalities. With more cars on the road, there would naturally be more accidents. So the seatbelt laws may be reducing fatalities, but this reduction is effectively cancelled out by the greater number of drivers.

This could resolve the discrepancy because the stimulus indicates that strict seatbelt laws tend to reduce fatalities, but if (D) is true, then these laws aren’t strictly enforced. The laws may not be reducing fatalities simply because most people are ignoring them.

This doesn’t explain why there wasn’t a decrease in fatalities. If anything, a correlation between not wearing seatbelts and deaths would give us more reason to expect laws requiring seatbelts to be effective, so the discrepancy becomes even harder to resolve.