Which language family do the Khmer and Mon languages belong to, unlike Thai?

• A Austroasiatic ✓
• B Tai-Kadai
• C Sino-Tibetan
• D Austronesian

Khmer and Mon belong to the Austroasiatic language family, distinct from Tai-Kadai, and are the substrate languages of much of mainland Southeast Asia, predating Tai migrations.