Authority-and-consent approach.
The key is to first decide whether this is a clinical autopsy or a medico-legal autopsy, because the two have completely different rules for who can order them.
An RTA fatality is an unnatural death, so it falls under the medico-legal category. Two facts in the stem confirm this: the doctor informed the police and sent the body to the mortuary rather than handing it to relatives.
For a medico-legal autopsy, consent of the relatives is NOT needed and, importantly, the doctor cannot self-initiate it. The examination is carried out only after the police or a magistrate sends a formal requisition/inquest report. In practice the Investigating Officer (police officer in charge of the inquest, typically a sub-inspector or higher) is the person who requests the autopsy and forwards the dead body with the relevant papers to the autopsy surgeon.
Eliminating the rest: the Public Prosecutor ($A$) and the Defence lawyer ($B$) are courtroom roles with no power over the conduct of a post-mortem; the Doctor ($C$) is merely the executor of the examination and must wait for the official requisition.
Correct option: D - Investigating officer.