What indicates that an aircraft is operating legally under VFR?

An aircraft operating legally under Visual Flight Rules (VFR) is recognized as squawking the transponder code 1200. This specific code is designated for VFR operations within the United States, indicating to air traffic control that the aircraft is operating under visual conditions rather than instrument conditions.

Using the code 1200 helps ATC manage airspace by identifying VFR traffic, facilitating a safer separation between different flight rules. When an aircraft transmits this code, it signals to ATC that the pilot can see and avoid other traffic and has visual references for navigation, differentiating it from IFR (Instrument Flight Rules) operations, where a different code would be utilized.

The other options do not indicate a legal VFR operation effectively. Code 1255 is not standard for VFR flights and does not denote visual operations. Squawking 7600 indicates a radio communication failure, which is not relevant to establishing VFR status. Lastly, only operating under an ATC clearance applies more to IFR, where clearance is mandatory for legal operation, rather than VFR, where pilots have more freedom as long as they adhere to visual regulations.