remember that, on "draw the conclusion" questions, you have to pick a choice that is RIGOROUSLY SUPPORTED by the information in the passage and that requires NO ADDITIONAL ASSUMPTIONS. none at all.

let's say the sales tax is s%. then if you spend c% of your income on consumer goods, then you spend s% of c% of your income on sales tax. notice, in particular, that this percentage does not change as a function of income; both rich and poor people pay s% of c% of their income in sales taxes.

the passage thus states that s% of c% for the poor is greater than s% of c% for the wealthy.
since s% is fixed (it's the statutory sales tax rate), this implies that c% for the poor is greater than c% for the wealthy, as stated in choice (b).

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(a) is wrong because nothing in this passage has anything to do with the relative numbers of poor and rich people.

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by the way, if you're going to play the "stick to things that are mentioned in the passage" game - a very, very good game to play on this question type - then (b) beats (a) anyway.

(a) mentions proportions of the population, a concept that's not mentioned in, or deducible from, the passage. therefore, (a) is outside the scope of the passage.

(b) contains only items that are actually in the passage: poor people, proportions of income, purchases of goods, wealthy people. (this doesn't guarantee that it's the correct answer, of course, but it's a good sign.)

THE CORRECT ANSWER DOES NOT HAVE TO CONTAIN EVERY CONCEPT THAT APPEARS IN THE PASSAGE!!
if you have any concepts that are not mentioned in the passage, then that's the death of whatever answer choice you're considering ... but few, if any, correct conclusions will contain all of the concepts from the passage.
remember that you have to pick a choice that makes NO ADDITIONAL ASSUMPTIONS. the more premises there are, the less likely that you'll be able to interconnect them all without making assumptions. therefore, it is highly unlikely that a correct conclusion will contain all the relevant concepts; therefore, the lack of "tax" is no reason at all to dislike this answer choice.