Final answer:
The best response when unsure about a customer's question is to admit lack of knowledge, apologize, and inform the customer when you will follow up. This maintains transparency and professionalism.
Step-by-step explanation:
To demonstrate competence when you don't know the answer to a customer's question, the best response would be option (d): Tell him or her you don't know, apologize, and say when you'll call back with the answer.
This approach shows professionalism and transparency, ensuring the customer feels heard and respected.
It provides you with the opportunity to research the answer and follow up with accurate information.
Additionally, it is important to provide realistic expectations and communicate clearly about when the customer can expect the follow-up, hence maintaining good customer service standards.