Understanding the argument -
A gas tax of one cent per gallon would raise one billion dollars per year at current consumption rates. - Premise/Opinion.
Since a tax of fifty cents per gallon would raise fifty billion dollars per year, it seems a perfect way to deal with the federal budget deficit. - Premise + opinion. But the premise is derived from the earlier premise. So the author assumes that if we raise the gas tax by one cent, we can raise 1 Bn; likewise, raising it to 50, 75, or 100 will lead to 50 bn, 75 bn, or 100 bn, respectively. So, the author is assuming that the consumption pattern will stay the same. What if the consumption reduces, then after increasing by 50 cents, it may for example lead to just two bn instead of 50? So, the author has made this assumption here.
Conclusion - "It seems to be a perfect way to deal with the federal budget deficit."
This tax would have the additional advantage that the resulting drop in the demand for gasoline would be ecologically sound and would keep our country from being too dependent on foreign oil producers. - ok, now it talks about additional benefits to strengthen our conclusion, which is "it seems a perfect way to deal with the federal budget deficit"
Drop-in demand for Gasoline would be
1. Ecologically sound
2. Reduce dependence on foreign oil.
So now it says that the demand will drop. Ok. So, earlier, we assumed that demand would remain the same. Then, we cited additional benefits to strengthen our conclusion that the demand would reduce, which contradicts what C highlights. The word "incompatibility" because, in conclusion, "it seems a perfect way to deal with the federal budget deficit." First, we assumed there would be no drop in consumption, but then we added a strengthening premise that there would be a drop in consumption. So, the flaw is that this assumption is incompatible with the premises stated in the argument.
Option Elimination. -
(A) The author cites irrelevant data. - No. Facts respected.
(B) The author relies on incorrect current consumption figures. - No. Facts respected.
(C) The author makes incompatible assumptions. - ok.
(D) The author mistakes an effect for a cause. - No. The effect of "reduction in Federal Deficit" is not mistaken as a cause that is not mentioned in the argument. An increase in tax collision can lead to a reduction in the Federal Deficit, but no specific cause of the Federal Deficit is stated in the argument.
(E) The author appeals to conscience rather than reason. - out of scope.