Who lives in glass house ___ not throw stones.

The correct answer is Should. This question tests knowledge of common English idioms and modal verbs. The proverb "Who lives in a glass house should not throw stones" means that people who have faults or weaknesses themselves should not criticize others. The modal verb "should" is used to express advice, recommendation, or moral obligation, which perfectly fits the context of the proverb.

• A: Has is grammatically incorrect in this context and does not convey the intended meaning of advice.
• C: Must implies a strong necessity or command, which is too forceful for the advisory nature of the proverb. "Should" is softer and more appropriate for giving counsel.
• D: None of these is incorrect as "Should" is the precise word.