Refer to the input text as context and select the correct relationship between 'Turkish Airlines' and 'Istanbul' from the available options.
Options: director/manager, industry, headquarters location, location of formation

Partnerships with airlines including Lufthansa and Poland’s LOT remain key even as Turkish Airlines builds its Istanbul base into a global hub with an eye to the success of Gulf operators Dubai-Emirates, Qatar Airways Ltd. and Abu Dhabi’s Etihad Airways PJCS, Kotil said.
location of formation