Cars are safer than planes. Fifty percent of plane accidents result in death, while only one percent of car accidents result in death. Which of the following, if true, would most seriously weaken the argument above?

The argument presented is that Cars are safer than planes supported with the data given. And we need to weaken this argument. 

In Option B, if we assume the number of plane accidents to be x. Then as per the data provided in the question, the number of deaths due to plane accident becomes x/2. 

And if we consider number of car accidents to be 100,000x. The number of deaths because of car accident becomes 1% of 100,000x, which is 1000x, which is way greater than that caused by planes(x/2). This clearly shows that cars are not safer than Planes. 

Hence, this weakens the main argument and is our final answer.