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