Option 1: Acetobacter is not used in milk curdling. Acetobacter is a type of bacteria that produces acetic acid, which is responsible for the fermentation of ethanol into vinegar.

Option 2: Leuconostoc is not used in milk curdling. It is a type of bacteria that is commonly found in dairy products and can contribute to their flavor.

Option 3: Bacillus is not used in milk curdling. Bacillus is a genus of bacteria that can be found in various environments and has different species, some of which may cause food spoilage.

Option 4: Lactobacillus is the correct answer. Lactobacillus is a type of bacteria commonly used in milk curdling. It produces lactic acid, which lowers the pH of milk and causes it to curdle. This process is important in the production of cheese, yogurt, and other dairy products.

Alert - correct answer should be option 4.