When should a pilot transition to the final approach during landing?

Transitioning to the final approach during landing should be done when the aircraft is abeam the runway threshold at the appropriate altitude. This positioning allows the pilot to correctly align the aircraft with the runway while ensuring a smooth descent. Being "abeam the runway threshold" indicates that the aircraft is at a perpendicular angle to the runway's approach section, which is crucial for establishing proper glideslope and final approach parameters.

Executing the final approach at this point ensures the pilot has sufficient time to configure the aircraft for landing, check the necessary systems, and make any adjustments needed to account for factors like wind. This procedure is standard practice in aviation, enhancing safety and efficiency during landing.

The other options provide scenarios that do not adhere to safe and standard landing practices, which is why they're not appropriate for final approach transition. Transitioning at cruising altitude, waiting for ground control instructions, or only doing so in inclement weather does not align with the established norms of flight operations.