What action should be taken when observing an invalid Mode C readout below FL180?

When an invalid Mode C readout is observed below Flight Level 180, the appropriate action is to issue the correct altimeter setting and request the pilot to verify their altitude. This procedure is crucial because an invalid readout could suggest that the aircraft's actual altitude does not correspond with what is being transmitted.

By providing the correct altimeter setting, you ensure that the aircraft is calibrated with the current pressure setting, which is essential for the accuracy of altitude reporting. Asking the pilot to verify their altitude additionally allows for confirmation that the aircraft is maintaining safe vertical separation from other traffic, which is vital for maintaining safety in the airspace. This proactive communication aims to clarify potential discrepancies and maintain operational integrity.

Other options, while they may seem actionable, do not appropriately address the safety concerns surrounding erroneous altitude information. Ignoring the readout or assuming it is correct could lead to dangerous situations with potential conflicts between aircraft. Therefore, the emphasis on verifying the altitude and ensuring correct altimeter settings is critical in air traffic management.