Final answer:
The correct answer is when the aircraft is on a published portion of the approach, as per the ATC clearance given to maintain 5000 until established on the approach.
Step-by-step explanation:
The question pertains to an approach clearance issued by air traffic control (ATC) to an aircraft intending to land at an airport without a control tower. According to the clearance "Bonanza 3bR maintain 5000 until established on a published portion of the approach, cleared VOR runway 13 approach, report FAF inbound", the pilot may begin descending when the aircraft is established on a published portion of the approach, which would typically occur after the aircraft reaches the Initial Approach Fix (IAF) and is aligned on the final approach course. Therefore, the correct answer is option (d) When the aircraft is on a published portion of the approach.