Explanation:

The question is about identifying which travel company declared bankruptcy in September 2019.

Option 1, Expedia, is incorrect because Expedia is still operating successfully as of today.

Option 2, Cox & Kings, though had issues with its financial stability, did not go bankrupt in September 2019.

Option 3, SOTC, is also functioning successfully and did not declare bankruptcy in September 2019.

Option 4, Thomas Cook, is the correct answer. Thomas Cook, the British global travel group, announced compulsory liquidation in September 2019, effectively declaring itself bankrupt due to its enormous debts, disappointing bookings, and high costs. Following this bankruptcy, tens of thousands of tourists were stranded worldwide.

So, the correct answer is Thomas Cook.