Responding to a pm:

The plan is to finance the improvements by increasing the toll on 2 highway bridges. What casts doubt on the plan?
The plan will be in trouble if the increase does not lead to increased collection.

Option (C) says that commuters can 'postpone' the effects of the increase. So it may take a little longer to get in the increased collection but you probably will see the increase.

Option (D) says that commuters switch to another route when toll is increased. So the overall collection may be actually less than before. This certainly casts doubt on the plan.

Hence, answer is (D)

for (C)
1st question: the fact that it happened once in the past cannot be taken as proxy that it will certainly happen again, right? something may have changed between then and now to force the people to stick to this route...
2nd question: we do not know what number "20 percent" represents, and neither do we know the exact budget which would "finance" the rail line. if a 20% decrease in traffic does happen, how can we say with certainty that the plan WILL fail?

can someone please clear this out?

According to the Tristate Transportation Authority, making certain improvements to the main commuter rail line would increase ridership dramatically. The authority plans to finance these improvements over the course of five years by raising automobile tolls on the two high-way bridges along the route the rail line serves. Although the proposed improvements are indeed needed, the authority’s plan for securing the necessary funds should be rejected because it would unfairly force drivers to absorb the entire cost of something from which they receive no benefit.

Which of the following, if true, would cast the most doubt on the effectiveness of the authority’s plan to finance the proposed improvements by increasing bridge tolls?

According to the Tristate Transportation Authority, making certain improvements to the main commuter rail line would increase ridership dramatically. The authority plans to finance these improvements over the course of five years by raising automobile tolls on the two high-way bridges along the route the rail line serves. Although the proposed improvements are indeed needed, the authority’s plan for securing the necessary funds should be rejected because it would unfairly force drivers to absorb the entire cost of something from which they receive no benefit.

Which of the following, if true, would cast the most doubt on the effectiveness of the authority’s plan to finance the proposed improvements by increasing bridge tolls?

The key to this question is to weaken the effectiveness of the FINANCING plan (i.e., tolls imposed on drivers for 5 years).

(A) Before the authority increases tolls on any of the area bridges, it is required by law to hold public hearings at which objections to the proposed increase can be raised.

(B) Whenever bridge tolls are increased, the authority must pay a private contractor to adjust the automated toll-collecting machines.

(C) Between the time a proposed toll increase is announced and the time the increase is actually put into effect, many commuters buy more tokens than usual to postpone the effects of the increase.

This answer choice says people can only "postpone" the effect. Even if many commuters buy more tokens to save in the short-term, the 5-year plan isn't really weakened.

(D) When tolls were last increased on the two bridges in question, almost 20 percent of the regular commuter traffic switched to a slightly longer alternative route that has since been improved.

If 20% of drivers choose to take the longer route and avoid tolls, then this certainly weakens the financing plan as it will fall short of its projection.

(E) The chairman of the authority is a member of the Tristate Automobile Club that has registered strong opposition to the proposed toll increase.