Change the given sentence into passive form. 

"The doctor had advised Anna not to eat chocolates."

In the active voice, the subject performs the action. The given statement, "The doctor had advised Anna not to eat chocolates", is in the active voice. Here, the subject of the sentence is the doctor who is performing the action "advised". We need to convert it into the passive form where the subject receives the action in the name of the verb. In the passive voice, the subject will be shown as affected by the verb. In the active voice, the sentence uses "had" and the past participle form of the verb. While converting it into the passive voice, "had been" and the past participle form of the verb should be used. Hence, option A is the right answer. Option B is wrong since simple past tense is used instead of the past perfect tense. Option C is wrong in its meaning itself. Option D is wrong since it uses past perfect tense.