Why not D or E ?
D) he mistakenly assumes that Giselle wants a sales tax increase only on gasoline
Giselle DOES want a sales take increase only on gasoline, so he doesn't make a mistake, hes actually correct.
(E) he makes the implausible assumption that the burden of increasing government revenues can be more evenly distributed among the people through other means besides increasing the gasoline sales tax.
basically saying, he assumes their is a better way of distributing the burden of revenue than just increasing gasonline tax. WHile he does make this assumption, it's outside the scope of what Giselle says, because Giselle DOESNT CARE about government revenue.