Analyze the relationship between 'Turkish Airlines' and 'Istanbul' within the context of the input sentence.
Options: creator, chief executive officer, headquarters location, parent organization

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.
headquarters location