Sales taxes tend to be regressive, affecting poor people more severely

When all purchases of consumer goods are taxed at a fixed percentage of the purchase price, poor people pay a larger proportion of their income in sales taxes than wealthy people do.

The obvious point here is why? Why should poor people pay a larger proportion? If everyone is paying say 10%, poor people are also paying 10% and so are rich. So if poor people are spending $100, they are paying $10 and if rich are spending $1000, they are paying $100. As a proportion of their income, they are paying same tax.
Until and unless, poor are spending more of their income on consumer goods and rich are spending less. I took an extreme example here to make sense immediately to me - say poor are spending the entire $100 they are earning on consumer goods and paying $10 tax (so 10% of their income) while rich are saving half their income and spending only half on which they pay 10% tax. So they are paying no tax on half of their income and hence as a proportion of their income, tax paid is much less than 10%.

(A) Poor people constitute a larger proportion of the taxpaying population than wealthy people do.

Not known. How many of the tax payers are rich and how many are poor is unknown.

(B) Poor people spend a larger proportion of their income on purchases of consumer goods than wealthy people do.

This matches the logic we discussed above. Poor people must be spending a larger proportion of their income on consumer goods to end up paying a larger proportion of their income as tax. Else everyone would be paying the same 10% of their spend as tax.

(C) Wealthy people pay, on average, a larger amount of sales taxes than poor people do.

Quite probably true but not implied from the argument. The argument doesn't tell us that wealthy spend more on consumer goods (and hence pay higher tax). The argument does not tell us that the poor spend $100 per person on consumer goods and wealthy spend $1000 per person. It just says that poor people pay a larger proportion of their income as tax.

(D) The total amount spent by all poor people on purchases of consumer goods exceeds the total amount spent by all wealthy people on consumer goods.

This depends on the number of poor, the number of rich, the avg spend of poor and the avg spend of rich. We don't know any of these figures and hence we cannot deduce this.

(E) The average purchase price of consumer goods bought by wealthy people is higher than that of consumer goods bought by poor people.

Again, probably true but the argument does not imply this. It tells us nothing about the average purchase price of consumer goods.

Answer (B)­