This sentence has a name (Bangkok) followed by a descriptive phrase about it, then the main clause continues, so spotting where the description begins and ends is the key.
Putting it together: R (so after leaving Bangkok) + Q (the largest urban area and primary) + S (city of Thailand and a place of unique beauty) + P (I picked up a liking for learning) + foreign languages.
So the correct answer is option B, RQSP.