Which of the following codes is for a city outside the United States?

The airport code PTY corresponds to Tocumen International Airport, which is located in Panama City, Panama. This makes it the correct choice for the question, as it specifically identifies a location outside of the United States.

For context, the other codes listed belong to airports within the United States: ONT stands for Ontario International Airport in California, SNA is the code for John Wayne Airport in Orange County, California, and MCO is Orlando International Airport in Florida. Each of these codes is associated with domestic flights and locations in the U.S., confirming that they are not applicable when identifying a code for a city outside the country.