The correct option is D a candy
Some verbs take two objects, a direct object and an indirect object. For example, in the sentence 'Shaima gave a pen', there is only one object 'pen'. In the sentence 'Shaima gave me a pen', (or 'Shaima gave a pen to me) there are two objects, 'me' and 'pen', where 'pen' is the direct and 'me' is the indirect object.
In the given sentence, "a candy" is the direct object of the verb "promised", hence option D is correct. "You" is the subject of the verb "promised" and "me" is the indirect object. Hence options A, B and C are incorrect.
Note: The direct object answers the question 'what?' and the indirect object answers the question 'to who/whom?'