Final answer:
To update that you are traveling to the client in Field Service for Dynamics 365, you should adjust the 'Booking Status' field, not Start Time, Actual Arrival Time, or System Status.
Step-by-step explanation:
When using the Field Service mobile app within Microsoft Dynamics 365, the correct field to update in order to indicate that you are traveling to the client is A. Booking Status. The booking status is used to track the progress of the work order from scheduled to completed, and changing it to a status like 'Traveling' or something similar (depending on the organization's custom statuses) will indicate that the field agent has begun their journey to the job site. This is distinct from fields like Start Time, which indicates when work is scheduled to begin, Actual Arrival Time, which should be updated upon arrival, and System Status, which represents the work order's overall progress through its lifecycle.