The city code for Mexico City is:

The city code for Mexico City is MEX. This code is designated by the International Air Transport Association (IATA) and is used by airlines and travel agencies worldwide to identify the airport serving Mexico City, which is officially known as Aeropuerto Internacional Benito Juárez. The MEX code is recognized and utilized in ticketing, flight information systems, and other travel-related services to ensure clarity and accuracy in flight operations.

In this context, MEX is the only option among the provided choices that corresponds correctly to the airport serving Mexico City. The other codes represent different locations: MEM is for Memphis, Tennessee; MCO serves Orlando, Florida; and MKE is the code for Milwaukee, Wisconsin. Each of these codes is unique to their respective cities and airports, highlighting why MEX is the correct choice for Mexico City.