What should you instruct an aircraft with an operable transponder, but not squawking correctly?

Instructing an aircraft with an operable transponder that is not squawking correctly to reset the transponder and squawk the correct code is important for maintaining effective communication and situational awareness. When an aircraft's transponder is functioning but not transmitting the correct code, it may lead to confusion for air traffic control and can hinder proper radar identification and tracking.

Resetting the transponder ensures that the aircraft is on the correct frequency and transmitting the right code. This is crucial for safety, as it allows for accurate identification of the aircraft and assists air traffic control in providing appropriate instructions and separation from other traffic. It enhances the overall safety and efficiency of airspace management.

The other options: changing frequency immediately, verifying the code first, or squawking 1200 (a common VFR code), may not directly address the issue of the transponder not squawking correctly and could introduce further complications in communication and tracking. Therefore, resetting the transponder and ensuring the correct squawk code is set is the most effective and direct approach.