When would you write a frequency in the C on a strip?

Writing a frequency in the C on a strip is specifically associated with indicating that the frequency being used is different from the standard or expected frequency for that phase of flight. This practice helps ensure clarity among controllers and can be critical in maintaining communication standards and avoiding confusion.

In situations where a frequency is not the one typically used or established for a particular flight, marking it in this manner serves as a clear alert to all personnel that they should be aware of this deviation. This can be important for ensuring that everyone involved in air traffic control is informed of any changes that could impact the safe and efficient operation of the airspace.

The other options do not fit this specific purpose; writing a frequency in the C is not typically required just because it’s the only frequency available, whether instructed by flight crew, or when transitioning to a lower altitude, as these scenarios do not inherently involve departing from standard frequency usage.