This is a tricky one! The key is to pay attention to the language:

A recent poll found that over 80 percent of the residents of Nalmed Province favored a massive expansion of the commuter rail system as a means of significantly easing congestion on the province's highways and were willing to help pay for the expansion through an increase in their taxes. Nevertheless, the poll results indicate that expansion of the rail system, if successfully completed, would be unlikely to achieve its goal of easing congestion, because _______.

(emphasis mine)

We need to choose an answer that indicates that the POLL ITSELF (an not outside circumstances) indicates that the plan will not work.

Here is the assumed line of reasoning in this argument:
Highways are congested --> Poll: people support building a commuter railway --> railway will ease congestion

That all seems reasonable so far, right? But there must be something missing in this argument. What would prevent the the railway, even if successfully completed, from easing congestion?
- what if people don't use the railway and continue to drive?
- what if the railway doesn't go to locations that people need to go? Or at times when people need to commute?
- what if congestion on highways isn't caused by commuters? And thus a commuter rail wouldn't affect it?
- what if building the railway caused some other problem that ended up increasing congestion?

Not only do we have to find an answer that indicates that the original line of reasoning has a flaw, but we need an answer that shows that the POLL itself reveals that flaw.

A. most people in favor of expanding the rail system reported less congestion during their highway commute as the primary benefit they would experience
If this is true, it shows that most people still plan to drive themselves! They see the commuter rail as a benefit that *other* people will use. If most people still plan on driving themselves, then the rail will be empty, and congestion will not be improved. Correct!

B. of the less than 20 percent of residents not counted as favoring the expansion, about half claimed to have no opinion one way or the other
This doesn't matter. As long as 80% are in favor, we would expect that the plan still holds: most people think that building this railway will ease congestion.

C. the twice-daily periods of peak congestion caused by people commuting in cars have grown from about an hour each to almost two and a half hours each in the past 20 years
This just gives us more explanation for how bad the congestion is. It doesn't give any reason to believe that building the railway will NOT make the situation better.

D. expanding the commuter rail system will require the construction of dozens of miles of new railbed
This would be relevant... if we were talking about the immediate affect on congestion. Construction often leads to congestion (although we don't know whether this construction will be near existing highways). However, our argument specifies the affect on congestion AFTER SUCCESSFUL COMPLETION. So this is irrelevant.

E. the proposed expansion to the commuter rail system will make it possible for some people who both live and work at suburban locations to commute by rail
This just gives more reason to believe that the railway expansion WILL ease congestion, not that it won't. This is the opposite of what we want.

The correct answer is A.