To act as pilot-in-command of an aircraft under 14 CFR Part 91, a commercial pilot must complete a flight review or proficiency check within the preceding:

To act as pilot-in-command of an aircraft under 14 CFR Part 91, a commercial pilot is required to complete a flight review or proficiency check within the preceding 24 calendar months. This requirement ensures that pilots maintain their skills and stay updated on regulations, procedures, and safety practices, which is crucial for safe operating standards.

The flight review must include at least one hour of ground training and one hour of flight training with an authorized instructor, covering specific maneuvers and maneuvers related to the pilot's certification. The purpose of this requirement is to uphold a level of proficiency and safety in aviation, reflecting the importance of regular training in the dynamic environment of flying.

When considering the other options, 12 months, 36 months, and 6 months do not align with the regulations outlined in Part 91. A flight review every year would be more frequent than required, while intervals longer than 24 months (such as 36 months) or shorter than 24 months (like 6 months) do not provide a sufficient framework for ensuring continued pilot effectiveness and adherence to the evolving standards of aviation.