An obento, or bento, is one of the oldest food traditions in Japan. Basically, it is lunch served in a box. Obento boxes have been part of Japanese society since the 5th century when people used to take a packed lunch with them to work. Bentos have become very popular since then and you can buy them almost anywhere nowadays — at theatres, airports and even at train stations.
The best heading for this paragraph is:
Answer: c) A long-standing Japanese tradition