Which statement is correct?

Which statement is correct?

a$.$ A good tax system cannot be simple, since the process of achieving equality amongst taxpayers requires a good deal of complexity

b$.$ A good tax system takes into account differences between individual taxpayers.

c$.$ A tax system should tax every individual taxpayer in the same way and at the same tax rate.

d$.$ Horizontal equity demands that different taxpayers in different circumstances should pay different taxes.