Could answer choice (D) be understood in another way?

If a tax has just been applied to a product AND consumers simultaneously have less money, then it seems that consumers would be even less inclined to purchase that item than they would be if the tax on that item had not been increased.

In other words, if you have less money and the price (with tax) of a product has NOT gone up, then you might be more likely to buy that product than if you have less money and the price (with tax) of that product HAS gone up.

Perhaps the problem with (D) involves the language used?
For one thing, it says "many" consumers, and we don't know whether people buying cigarettes fall into that group. In contrast, (B) refers to "consumers." Can (B) be understood to refer to ALL consumers?

Second, does one have to make an assumption (see the use of the word "might" above) about people's potential buying behavior if they have less money and the tax on an item goes up? They might have less money, but they might still buy an item on which the tax has been increased. Or they might not.

i think you're not paying enough attention to the actual issue in the argument.
the issue is this: did people buy fewer cigarettes BECAUSE the after-tax price decreased, or for some other reason?
anything pointing to "...because the price decreased" is a strengthener.
anything pointing to "... for some other reason" is a weakener.

if people's income went down -- even if it were only some people's income -- then that definitely points to "some other reason" (where the other reason is that people had less money to spend!)
so, that choice weakens the argument. that's exactly the opposite of what we want.




on strengthen/weaken assumptions, you are supposed to make the most reasonable / most common-sense assumption about the statements.
for instance, if you see "At the emergency room, patient X had injuries characteristic of someone hit by a large vehicle", then you go ahead and assume that patient X was hit by a large vehicle. you do not go the route of, "well, maybe something else happened that left the same type of injuries".

this is why there are so many of these problems on the test (at least half of all CR problems are strengthen/weaken) -- because, above all else, the test writers want to make sure that you can interpret situations in a real-world / common-sense sort of way.
note: if you come from a tech background, especially if you are a programmer (in which the thought process is "let me think of anything that can possibly go wrong with this code, no matter how remote"), it's going to take some effort to move back to thinking in this sort of way.