An effective commuter train tunnel between the counties would require major investment in mass transit within both counties. - Weakens
The majority of all vehicles on the nation’s freeways are traveling from one state to another. - General, so out of scope.
60% of the cars on Freeway Z are driven by people who live in County Y and work in County X. -Correct. This would mean a constant source of traffic and hence a good purpose for the plan.
Many new freeways are being built in areas that are presently served by commuter trains.- General view
A large proportion of the vehicles on Freeway Z are commercial trucks carrying transcontinental shipments. - Irrelevant.
IMO, ans is C.