Option 1: Raagi - Raagi is not commonly used in preparing custard powder. It is a type of millet that is often used to make porridge or flour.

Option 2: Wheat - Wheat is not commonly used in preparing custard powder. It is primarily used to make bread, pasta, and other baked goods.

Option 3: Maize - Maize, also known as corn, is commonly used in preparing custard powder. It is ground into a fine powder and mixed with other ingredients to create the custard powder.

Option 4: Rice - Rice is not commonly used in preparing custard powder. It is a staple grain that is used in a variety of dishes, but not in custard powder.

Therefore, the correct answer is option 3: maize.