Final answer:
The most useful question to ask is "What is the brand and model of your food mixer?" as it helps determine attachment compatibility.
Step-by-step explanation:
The most useful question to ask a customer who needs an attachment for a food mixer is "What is the brand and model of your food mixer?" This information is crucial in determining the compatibility of the attachment with the food mixer. Different food mixers have different attachment systems, so knowing the brand and model is essential for finding the right attachment.
Asking about the warranty, purchase location, or age of the food mixer may provide helpful information in certain situations, but they are not as directly relevant to finding the appropriate attachment.