What should be done if the Mode C readout remains invalid for an aircraft below FL180 after issuing a correct altitude?

If the Mode C readout remains invalid for an aircraft below FL180 after the pilot has confirmed they are at a correct altitude, the most appropriate action is to instruct the pilot to contact their operations supervisor. This step is crucial because continued invalid altitude information can pose operational risks, including potential conflicts with other aircraft, especially in controlled airspace.

By contacting their operations supervisor, the pilot can receive guidance and support from the airline's operations team. This team can assist in determining whether the aircraft needs to adjust its altitude, change its routing, or take other necessary measures to ensure safety. This action helps to ensure that the aircraft is taking appropriate measures while allowing for effective communication between the crew and their operational support team.

The alternative options do not address the specific operational context of an invalid Mode C readout adequately, potentially leading to unnecessary distress for the pilot or other complications in air traffic management.