Why do they ask how many hours a week I prefer to work and what is the best answer?

They are trying to establish your availability and whether you are prone to overworking or are perhaps lacking motivation. The best answer will indicate that you are able to keep a good balance between family life, work, friends and interests. People who overwork can burn out and don’t have enough free time to recharge their creativity. People who only work to get an income are not committed. An example of a good answer will be:'I try to keep a good balance between work and my family life. I believe that if I do my best while at work, the normal hours should be enough to complete most projects. That being said, I don’t hesitate to put in extra time to get a job done in time and to ensure a high standard. I prefer to start earlier than my colleagues since it provides me with enough time to take care of the administration and planning of the day so that I can perform at my best.'