What must passengers do if carrying more than 100ml of liquid?

Passengers carrying more than 100ml of liquid must pack it in checked luggage because security regulations limit the amount of liquids passengers can bring in their carry-on bags. The 100ml rule is primarily a measure to ensure safety within the cabin, as larger quantities of liquid could potentially be used to create harmful substances. By placing liquids over this limit in checked luggage, they remain out of reach during the flight, addressing safety concerns while still allowing passengers to travel with the items they need.

Other options, such as declaring it at check-in, throwing it away before security, or attempting to store it under the seat, do not comply with airline regulations and could lead to complications at the security checkpoint. Thus, packing liquids over the specified amount in checked luggage is the appropriate course of action to adhere to security guidelines.