(B) says, "The proportion of serious automobile accidents involving child passengers has remained constant over the past 8 years."
(B) looks great, as it helps to strengthen the argument because it keeps the proportion of kids who have been in serious car accidents constant. If there were actually fewer children involved in car accidents, then that could potentially explain the smaller rise of child fatalities during the last 8 years, so (B) eliminates this alternative possibility, thereby strengthening the argument.
Does this make sense? Let us know if you have any other questions!