Final answer:
The best word to describe a very talkative individual is 'garrulous.' This adjective fits the context of the sentence provided, indicating someone who talks a lot.
Step-by-step explanation:
The question asks for an adjective to describe a very talkative person who seated next to Andrew for a 12-hour flight. The correct answer is 'garrulous,' which means excessively talkative, especially on trivial matters. Therefore, the sentence would be completed as: 'Andrew had the unfortunate luck of being seated next to a garrulous young woman for his 12-hour flight.'