To answer this question, we need to understand the Greek characters mentioned in the options and their meanings:
Option A) pie - "Pie" is not a Greek character.
Option B) mu - "Mu" is the 12th letter of the Greek alphabet and has no negative connotation attached to it. It is not an undesirable Greek character.
Option C) zeta - "Zeta" is the 6th letter of the Greek alphabet and also does not have any negative connotation attached to it. It is not an undesirable Greek character.
Option D) stigma - "Stigma" is a Greek character that represents the number 6 in the Greek numeric system. However, in English, the word "stigma" is often associated with a mark of disgrace or shame, making it an undesirable Greek character to be attached to.
Therefore, the correct answer is option D) stigma. This option is correct because "stigma" is an undesirable Greek character if attached to you due to its negative connotations in the English language.