What can be added to a Form D that is already in effect?

The correct choice indicates that a Form D that is already in effect can only be amended to include information related to cancellation or confirmation of track clearance. This is consistent with the operational procedures that govern the use of Form D, as the primary purpose of such amendments is to ensure safe and clear communication regarding the status and availability of tracks for the movement of trains.

Adding new crew member instructions, additional details about crew training, or specific speed regulations would typically be outside the scope of a Form D amendment that is already in effect. Such changes usually require a separate procedure, as they involve training and operational policies rather than just immediate operational safety concerning track usage. Therefore, the focus of amendments to Form D is limited to matters directly related to the track's status to maintain clarity and safety in operations.