To solve this problem, we need to determine which vitamin's amount is increased when milk is converted to curd.$\newline$Let's analyze the options:$\newline \bullet$Option 1: Vitamin B$_{12} \newline$ - The conversion of milk to curd involves the fermentation process by lactic acid bacteria.$\newline$ - These bacteria can synthesize Vitamin B$_{12},$ thus increasing its content in curd compared to milk.$\newline \bullet$Option 2: Vitamin A$\newline$ - Vitamin A content is not significantly affected by the fermentation process.$\newline \bullet$Option 3: Vitamin D$\newline$ - Vitamin D levels are not increased during the conversion of milk to curd.$\newline \bullet$Option 4: Vitamin E$\newline$ - Vitamin E content remains largely unchanged during the fermentation process.$\newline$Based on this analysis, the conversion of milk to curd primarily increases the amount of Vitamin B$_{12}. \newline$Therefore, the correct option is Option 1.