Sticking with the original options…
You can rule out broth right off the bat; a broth is a liquid prepared through cooking to demonstrate a particular flavor(s) (no, pasteurization of your milk doesn’t count).
A sauce is a preparation meant to act as an accompaniment to a dish. Milk is not “prepared” in this sense (again, pasteurization doesn’t count). Though it could be argued that it is an accompaniment and the addition of sugar, for example, makes it a preparation so it can, under the correct circumstances, be a sauce.
A beverage is any liquid suitable for drinking. So long as your cereal doesn’t create an inability to drink your milk it is a beverage.
Therefore for your milk while containing cereal is circumstantially a sauce, a beverage, both, or simply a liquid.
On the other hand you could just say that if it is agreed that milk by itself is a beverage. Adding cereal to it doesn’t change the nature of the milk in any meaningful way therefore it is still milk and as such still a beverage.