This question is asking you to pick the answer choice which most strengthens the argument. This means that there could be multiple answer choices which seem to strengthen the argument, but you're looking for the one that does so the most.
The argument tells us that using car seats save lives because even though the number of kids who have died in car crashes has gone up by 10%, the total number of accidents has gone up 20%. This implies that the number of kids who died should have gone up by 20% as well.
Answer Choices:
(A) is incorrect because it is irrelevant. The argument is about kids under the age of 4, we are not concerned with them at age 5.
(B) is correct because it fills in a logical gap in the argument. We don't know that the total number of accidents always include children. So, the argument could be comparing apples and oranges. But, Answer (B) clears that up and tells us that the 10% increase in child deaths is indeed less than the 20% increase that could have been expected.
(C) is incorrect because it is irrelevant. The number and duration of trips does not affect the argument.
(D) is incorrect because it is irrelevant. The argument only cares about the age range of children under 4, we don't need to break that down any farther.
(E) is incorrect because it is irrelevant. We are interested in children under 4 not adults.