For what is required by the question and the options of answers provided, I assume the underlined word was '<u>but</u>'. If that's the case, then the best option would be letter B. Both 'however' and 'but' are adversative adverbs, that is, transition words with an adversative implication - They connect two informations that generally contradict one another (like in the example above). The words 'although', 'nonetheless' and 'nevertheless' have the same function, <em>although</em> used in different situations.
I think the correct answer from the choices listed above is option B. The transition word that would best replace the underlined word would be however. It is <span>used to introduce a statement that contrasts with or seems to contradict something that has been said previously</span>. Hope this helps.
