IMO, E is correct.

a) you should included a self-addressed, stamped envelope
Wrong.
- Should + past tense ==> incorrect.

b) it is recommended that you would be including a self-addressed, stamped envelope
Wrong.
- The structure should be parallel: when you send....., you should............ ==> "it is recommended that" is not appropriate.
- "Would be + Ving" is the past tense of "will be + Ving" ==> you mention a thing in the past. For instance: I didn't go to the party last night because I would be going to class. ==> B changes meaning.

c) a self-addressed, stamped envelope is that which you should be including
Wrong.
- "a self-addressed, stamped envelope" ==> Plural ==> "are" should be used.
- "that which" is incorrect grammar.

d) you might should have included a self-addressed, stamped envelope
Wrong.
- "should have included" is used to indicate an obligation that was supposed to have occurred in the past.
- "might should" is incorrect.

e) you might ought to include a self-addressed, stamped envelope
Correct.
- might ought to = might have to ==> mentions uncertainty
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No examples come to mind that include 'might ought'. 'Should' yes, but not 'might ought'. Also, the GMAT doesn't use 'you' or 'I' or 'we' in SC.

KW
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No examples come to mind that include 'might ought'. 'Should' yes, but not 'might ought'. Also, .

KW[/quote]

isn't it is too precise to say that :
the GMAT doesn't use 'you' or 'I' or 'we' in SC

SKM
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Might ought is not ungrammatical, but it sure sounds folksy. While having a question with this usage is useful to members, just to bring it to notice, I hardly think anyone is going to see this on the actual exam.

C is passive and B’s tense is clearly wrong, so they are both gone. ‘might should have’ in D does not make sense. The tense in A is also incorrect, its past when this is advice for the future ( I misread that when first reading, so I got confused). This leaves E, even though it is wordy, as the best option.