We don’t know if “many areas” includes the transportation official’s city or any of the other cities that she references. Because of this, (A) is too vague to strengthen her conclusion.

This weakens the argument by pointing out another factor that could be contributing to the ruts in the roads. If the ruts decrease as large truck traffic decreases, it’s more likely that large trucks also contributes to the ruts in the roads.

This strengthens the argument by eliminating another possible cause of the ruts. If places with studded snow tires but negligible large truck traffic also have similar ruts in the roads, then the large truck traffic is likely not contributing to the ruts.

The transportation official’s argument blames studded snow tires for the ruts, and we don’t know whether the cities mentioned in (D) also have studded snow tires. Thus, (D) is irrelevant.

Irrelevant— the transportation official’s argument only addresses those places that do allow the use of studded snow tires and the effects of those tires on the roads.