What is the relationship between 'Turkish Airlines' and 'Istanbul' in the context of the input sentence.
Options: headquarters location, location of formation, industry, director/manager

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