A、Supper. B、Lunch. C、Breakfast. D、Dinner. D 根据“For most people the main meal of the day is called dinner．”，因此答案是D。

In different parts of Britain, people call their meals by different names. It’s very complicated. For most people the main meal of the day is called dinner. But some families eat this at mid-day, others eat it in the evening. People who have dinner in the evening have a smaller meal called lunch at mid-day, and people who have dinner at mid-day usually have another meal which they call tea between five and six, or they have supper a little later.
    For most people, breakfast is a bowl of cereal followed by toast and jam with a cup of coffee or tea. Lunch is usually a very quick meal; most people have a sandwich or a light snack. Dinner may have two or three courses. First a starter, for example soup, then a main course with meat or fish and finally a dessert.
    It is probably tree that British food does not have a very good reputation among people from overseas. In general, though, it has improved considerably in recent years.