Answer:
Los cereales son mi comida preferida para el desayuno.
Translation:
Cereals are my favorite breakfast food.
Step-by-step explanation:
The correct option here is LOS. Articles in Spanish should always agree on number and gender with the noun.
Las should only be used when the noun is plural and feminine; and cereales is plural and masculine. For this same reason, unas would also be a highly incorrect article for the noun.
Unos, however, agrees with both the number and the gender and the noun, but it doesn't sound as appropriate for this sentences as Los does.