What is a common expression of gratitude in Thai?

The expression "Khob khun" is indeed a common way to express gratitude in Thai. It translates to "thank you" in English and is used in everyday conversations to show appreciation. In Thai culture, expressing gratitude is important, and using this phrase can help build rapport in various social interactions, whether formal or informal.

The other options serve different purposes: "Sawadee" is a greeting similar to "hello," "Chai yo" is often used as a cheer or encouragement, and "Sabai dee" conveys that one is well or in good health. While these phrases are important in Thai communication, "Khob khun" specifically denotes thankfulness and is the correct expression of gratitude.