We want to find the option the most strongly support the experts' contention about the effect of the proposed automobile tax on Birangi's air-quality problem.

Premise: A 50 percent tax on automobiles in Birangi would halt the rapid increase of automobiles on Birangi's road, and thereby slow the deterioration of Birangi air quality.

Conclusion: Most experts agree that such a tax would result in fewer Birangians buying new vehicles and gradually reduce the number of automobiles on Birangi's road, they contend that it would have little impact on Birangi's air-quality system.

Option A - Incorrect
We cannot validate this claim from the passage if automobile emissions are the largest single source of air pollution as there are many sources of air pollution.

Option B - Incorrect
The passage did not lay any emphasis on how the proceeds from the new tax will be spent. As such, this claim is irrelevant to the passage main focus.

Option C - Incorrect
This is out of context in respect to the passage main focus.

Option D - Correct
This claim proves a point to strengthens the experts' contention. While the reduction in the number of automobiles in Birangi will make it less fuel-efficient, it also contributes a great significance to air pollution as they age. So with this, the 50 percent tax on new automobiles will only have a massive reduction on the number of automobiles on Birangi's roads but little impact on the deterioration of Birangi's air-quality problem.

Option E - Incorrect
This is outside the scope of the argument. The argument is not concerned with the scraping of automobiles.