When should you write a frequency in the 'C' block on a strip?

Writing a frequency in the 'C' block on a strip is appropriate when the transmission is other than standard. This indicates to the controller that the aircraft or the situation requires special attention or adherence to protocols that differ from normal operations. The 'C' block is specifically designated for non-standard frequency assignments or when an aircraft is instructed to use a frequency that is not typically associated with the sector or flight path it is operating on.

This practice helps ensure clear communication and proper coordination, especially in busy airspace where unique situations might arise that necessitate a different frequency for the safety and efficiency of air traffic operations. Other options might suggest scenarios related to standard procedures, emergencies, or supervisor authorization, but these do not capture the unique context that necessitates a non-standard frequency assignment.