The correct answer is D. Let not the poor be insulted.
A. Let the poor not to insult is grammatically incorrect. The infinitive “to insult” cannot be used with the modal verb “let.”
B. Let the poor to be not insulted is also grammatically incorrect. The negative particle “not” cannot be used twice in the same clause.
C. Let us not insulted the poor is grammatically incorrect. The verb “insult” must be conjugated in the third person singular, since the subject is “the poor.”
D. Let not the poor be insulted is the only grammatically correct option. The negative particle “not” is used correctly, and the verb “insult” is conjugated in the third person singular.
In addition to being grammatically correct, option D is also the most concise and direct way of expressing the sentence “Do not insult the poor.”