Final answer:
Flat whites and Frappuccinos can be made with whole milk, skim milk, almond milk, or soy milk.
Step-by-step explanation:
The milk used in flat whites and Frappuccinos depends on the personal preference of the customer. By default, Starbucks usually uses whole milk for these beverages, as it provides a creamier and richer taste. However, customers can also request alternative milk options such as skim milk, almond milk, or soy milk to suit their dietary preferences or restrictions.