This is a loaded question. If you don't clarify the question, you could be talking about anything from the person who continuously asks questions to the person who instigates' trouble. If the interviewer just responded that any challenging co-worker. As a previous Amgen employee, the Amgen culture embraces listening. On the flip side, it would only be polite to hear what the challenging co-worker has to say. Then you should acknowledge the question, assess for relevance to the task at hand or if in a team meeting - give it an opportunity to breath with the team or yourself. If the thought or recommendation is good, then proceed to implement the recommendation(s). If it's someone who pontificates, give the person a 30-second moment of fame, then talk with them after the meeting about what was said. They just might not realize that they were rude or have a behavior that delivers the wrong perception to the majority in their presence. The key to answering this question is listen, acknowledge, assess, and praise the person for speaking up, if a negative perception - speak with the person in private for coaching.