The correct option is C breakfast
The object is the person or thing that receives the action in a
sentence. In the given sentence, "breakfast" is the recipient of the
action "ate", hence it is the object and option C is correct. "Mother" is
the subject and "ate" is the verb, hence options A and B are
incorrect. "In silence" is a prepositional phrase, it is not the recipient of the action "ate", hence option D is incorrect.