The correct option is C needn't be
"Needn't" is used to talk about an action that can be done but is not necessary a rule. The context refers to the action (being polite in my house) that can be done, but is not necessary as rule. Thus, option C is the correct answer.
"Mustn't" in option A is used say what is definitely unnecessary and to give strict orders to ourselves or other people to not do that action. However, this is incorrect as just saying what one likes is an option in the speakers house. Thus, this is incorrect in the given context.
"Shouldn't and ought n't", in options B and D indicate pieces of advice.
Thus, option C is the correct answer.