Here's the answer:
“お一人ですか？”
(o hitori desu ka?)
Watch a real native speaker say it:
Here's how it sounds in a textbook:
You could also say:
- あなたはお一人ですか？
- あなたは一人ですか？
Fun fact: the hundreds of thousands of people learning Japanese with Memrise get this phrase correct 94.48% of the time!
