A transitive verb is a type of verb that demands to be completed by a direct object, as opposed to intransitive verbs that don't have to have an object. For example: She is reading a book (where reading is transitive because it demands the object a book), as opposed to: He is sleeping (where the verb sleeping is intransitive and doesn't need an object.)
Having that in mind, the correct answer is B. Mom always eats a whole-grain cereal for breakfast.
The easiest way to find an object is to ask the question - what? What does mom always eat? - A whole-grain cereal for breakfast (direct object, meaning that the verb eat is transitive)