This is an Evaluate question, so the most useful question is the one whose two possible answers would push the argument's force in opposite directions. The council member grants that the average downtown car commute is three minutes shorter than a year ago, but argues that the parking-to-lane conversions, not the rapid-bus line, may be what produced that improvement. The question that matters most is how much of the three-minute improvement the conversions alone could explain.
(D) does exactly this. If the streets where parking became traffic lanes carry enough car traffic that the conversions alone could account for the full three-minute improvement, the bus line is not needed to explain the figure. If those streets carry too little traffic for that, room remains for the bus line to have contributed. Because a yes answer and a no answer pull the argument in opposite directions, this question is genuinely useful to ask.
The closest wrong answer is (B), which returns to the three-minute figure itself, the very number the council member already grants and argues is inconclusive as to its cause. Re-confirming a figure both sides already accept cannot help weigh which cause produced it. The decisive difference is that (D) sizes up the competing cause, while (B) only re-checks the already-granted effect.
(A), (C), and (E) raise the line's finances, rider comfort, and results in other cities. None of these can separate the bus line's contribution from the parking conversions, the split the council member's argument turns on. Only (D) sizes up that competing explanation.