Answer: B

This is an assumption question. Jonathan points out an increase in traffic through a major intersection and complains about a likely increase in fumes and congestion. Daria's response, though it isn't stated explicitly, is that 1,000 vehicles per day isn't that much of an increase. Her evidence: She sees enough vehicles go through the intersection to suggest that 1,000 is a typical hour. The underlying assumption is that her experience is, in fact, typical--that 1,000 cars usually go through the intersection per hour. That may suggest that the increase in congestion and fumes is slight, on a percent increase basis.

(A) Daria isn't considering the pattern of the traffic light, just the vehicle volume.
(B) This is correct. This is her assumption – that 50 vehicles per three minutes is typical. If it is, she's right to question Jonathan's concern. If it is unusually high, her argument is weak.
(C) This is something like an assumption of Jonathan's argument, not of Daria's.
(D) Daria is questioning Jonathan's assessment, not agreeing with it, as this choice would suggest.
(E) This is far outside of the scope, which is specific to the number of cars passing through the intersection over various periods of time.