A good way to avoid confusion here is to separate Order VI Rule 16 from two other CPC powers it is often mixed up with in exam questions, namely rejection of a plaint and amendment of pleadings on a party's own application.
Once Order VI Rule 16 is correctly separated from plaint-rejection and party-initiated amendment, its own scope confirms that the court may strike out the unnecessary and prejudicial pleadings at any stage of the proceedings.
Thinking about this from a case-management perspective, namely why a court would even want this power, also points to the same answer.
Efficient case management therefore calls for a targeted, promptly available power, and that is exactly what Order VI Rule 16 provides: the court may strike out such pleadings at any stage of the proceedings, which is the correct answer.