Correct Answer: I was told by the doctor to start taking medicines prescribed by him.

Solution : The correct choice is the first option.

Passive voice is the voice in which the object experiences an action rather than the person who performs the action. Hence, the object in the active sentence becomes the subject in the passive voice.

The structure of the passive voice in the case of the simple past tense is: "Singular object + was + past participle form of the main verb + prepositional phrase."

Therefore, the correct sentence would be: "I was told by the doctor to start taking medicines prescribed by him."