The correct answer is Option B.
A proper noun is a specific name of a person, place or thing. This name is exclusively used for that person, place or thing.
In the given sentence BANGKOK is the name of a particular city. Therefore it is the proper noun in the given sentence.
Hence Option B is the correct answer.
Options A and C are incorrect because they are common nouns i.e. they do not point to or name anything specific
Since Option B is the correct answer, Option D is incorrect.