Final answer:
The correct phrase to use when discussing clothing size with a customer is to ask them what size they believe they are, which respects their autonomy and avoids making assumptions.
Step-by-step explanation:
When discussing clothing size with a customer, it's important to maintain a professional and respectful tone. The appropriate phrase to use from the options provided would be C) "What size do you think you are?" This question is polite and places the emphasis on the customer's own perception of their size, which respects their autonomy and comfort. Options A), B), and D) make presumptions about the customer's size and could potentially be insensitive. Instead, prompting the customer to provide their size or if they're uncertain, offering to assist with measurements, can create a positive customer service experience.