The dialogue on Varna between king Avantiputta and Kachchana, a disciple of Buddha, appears in the Majjhima Nikaya, which is one of the collections of discourses in the Pali Canon, the scriptures of Theravada Buddhism. The Majjhima Nikaya consists of 152 suttas or discourses, and it is the second of the five Nikayas or collections in the Sutta Pitaka, which is the basket of discourses.

Option 1: Majjhima Nikaya - This is the correct answer. It is one of the major collections of discourses in the Pali Canon, containing various teachings and dialogues.

Option 2: Samyutta Nikaya - This is incorrect. The Samyutta Nikaya is another collection of discourses in the Pali Canon, but it does not contain the dialogue on Varna.

Option 3: Anguttara Nikaya - This is incorrect. The Anguttara Nikaya is another collection of discourses in the Pali Canon, but it does not contain the dialogue on Varna.

Option 4: Ambattha Sutta - This is incorrect. The Ambattha Sutta is a specific discourse within the Digha Nikaya, another