Answer:
The correct answer is letter "D": discretionary.
Step-by-step explanation:
Tax structures are divided into three categories. Each one of them affects directly people with low, medium, and high incomes. Regressive taxes might have a bigger impact on people with low income while proportional taxes affect people in the medium range. Finally, progressive taxes impact directly on people whose income is higher.
In this sense, "discretionary" is not a type of tax structure.