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