From what viewpoint is a sales tax most likely to be considered regres

From what viewpoint is a sales tax most likely to be considered regressive?

a. If one considers lifetime consumption as equal to income.
b. If one considers that many goods consume by low-income individuals are exempt from taxation.
c. If one considers it as a consumption tax on current income.
d. If one considers it as equal to a tax on wealth.

ANSWER

c