The correct answer is A) proportionally.
According to the chart, the citizens are being taxed proportionally.
When using proportionally tax, the government charges the same percentage of tax to taxpayers, independently of the individual income. The tax is charged a flat rate. So many people identify this tax with the name of flat rate. The graph clearly shows that citizen A has an income of $35,000 and citizen B has an income of $72,000, and both citizens are charged 11% of tax.