OG ans is A but the correct answer is D. If anyone can prove me wrong, let me know.
(A) The government of Maravia is in the process of building a significant number of roadways.
(D) Ninety percent of the population of Maravia is very prosperous and has a substantial amount of disposable income.
OG ans explains for A states new road does not necessarily equates more car usage, they can render unused, while D they suggest people are wealthy hence new tax will have less effect on people.
Both A and D require assumption / inference, however A contains a better assumption than D. It is because: a government's decision to build new roadways would always be based on some level of consultation / demand-supply analysis, as a result it is unlikely to build empty road and waste tax payer's money, time, resource and efforts; note also the phrase "building significant number of roads" - let's say at least some car will be added to the road.
On the other hand one, ans D require assumption of human psychology that if you are wealthy you are less sensitive to tax. This assumption is based plainly on stereotype and can be arbitrary, and I know very well, from my personal experience / studies, that wealthy people are wealthy because they are smart / sensitive about money. elasticity and people's reaction to tax pretty much depends case by case. Hence, unlike ans A that has an assumption that is based on common government practice / logical rationale, ans D is more susceptive to challenge. A is a better argument.