Step 1: Understanding the Concept:
The question asks for the common regional name for sweetened dahi (Indian yogurt).
Step 2: Detailed Explanation:
(A) Misti dahi: This is the correct term. Originating from Bengal, "Misti" means sweet and "Doi" (or Dahi) means curd/yogurt. It is a traditional sweetened yogurt made by fermenting sweetened, often slightly concentrated, milk. It has a characteristic pinkish-brown color due to the caramelization of sugar.
(B) Blue dahi: This is not a recognized term for a dairy product.
(C) Rasmalai: This is a dessert made from chhana (acid-coagulated cheese) patties soaked in sweetened, thickened milk (rabri). It is not a type of dahi.
(D) Kheer: This is a type of milk pudding, usually containing rice, vermicelli, or tapioca. It is not a fermented product.
Step 3: Final Answer:
Sweetened Dahi is famously known as Misti dahi.