Heres the explanation:

Option A states that both regular patrons and occasional patrons of the restaurant will be impacted in a similar way on seeing the 'Thank You' note. However, the impact is not mentioned clearly and hence this is out of scope.

Option B states that seeing the 'Thank You' note regularly would not lead restaurant patrons to revert to their earlier tipping habits. Thus, this option eliminates the possibility that would weaken the argument. In other words, using the assumption negation technique, we can see that if the patrons revert to their earlier tipping habits on seeing the note regularly, the average income of the servers would not be higher. This contradicts the original argument which concludes that higher tipping on seeing the 'Thank You' note will lead to a higher income for the servers. So this is the correct answer choice.

Option C seems to be a bit far fetched because in order for this option to be correct, we need to assume that the patrons would tip higher as they would think, on seeing the thank you note, that the tip is a significant part of the servers income.

Option D seems to be out of scope as we are not concerned with how expensive or less expensive a restaurant is.

Option E seems to be irrelevant as well because it adds on to the given premise that tip was high when the bill was accompanied by a Thank You note.

Hope this helps.

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A, D and E are out of scope.
C is wrong because we would assume that every patron would contribute a higher tip upon reading the thank you note, which is not the case.
B is correct as the argument relies upon this assumption to get an increased income from tips.
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+1 B
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+1 B
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Clearly B.

Sudish's explanation is a nice way to understand the solution to this problem.