The passage tells us that more people on Earth die in automobiles than on motorcycles. It concludes that automobiles are more dangerous.

The issue in this argument is a relatively common one on the GMAT, so we may recognize it right away - there are likely to be more individuals who ride in automobiles than on motorcycles. This means that there are likely to be more people who die in cars as a whole.

For instance, say there are 500 people who rode in cars and 200 people who rode on motorcycles in a given day. If 50% of all people who rode in cars and 100% of all people who rode on motorcycles died, we would say that the motorcycles were more dangerous. However, 250 people would have died in cars, while only 200 people would have died on a motorcycle. So in this example, even though more people died in cars, motorcycles are more dangerous - the proportion of people who died using each vehicle to the number of people who used each vehicle is more important than the total number of deaths for each vehicle.

So we need to compare the ratio of deaths in cars to number of people driving in cars to the ratio of deaths on motorcycles to number of people driving on motorcycles to determine whether or not cars are more dangerous. This leads us straight to answer choice D, which is the correct answer.