What is the ruling on saying "I am tired" or "busy" to one's mother politely when she asks for something to be done?

It is permissible sometimes to politely and gently apologize to the mother for not fulfilling her command due to fatigue or similar reasons. If the mother accepts the apology, then there is no blame on the child. However, if the mother does not accept the apology because she needs the matter fulfilled, then it is obligatory for him to obey her, even if it entails potential hardship, as long as there is no harm to him in obeying her.