What class of food does orange juice belong to?
Many foods are classified by group in their natural and unaltered state. Essentially, if looking at food groups in a universal way, many foods are still considered a part of the food group they are thought to belong to either cooked or raw. That said, if a food is altered too much, the altering may force the food to be disassociated with a group given the majority of the food's natural state has been altered too much.
If the orange juice is freshly squeezed and nothing has been added, orange juice belongs in the fruit and vegetable group. Unfortunately, many times when fruit juices are made, the juice is altered (through the adding of preservatives and sugars).
Therefore, if a juice is bottled, and not squeezed fresh at home (or in a restaurant), the juice does not belong to any food group. The orange can no longer be considered a 100% true orange and, therefore, do not qualify as a true fruit.
I am pretty certain the question refers to which group of organic molecules does the food contain. In that case, oranges contain carbohydrates, due to the sugars inside the fruit. It is a good source of other nutrients like Vitamin C, A, antioxidants and fiber if there is pulp in the juice. However, the group to which it belongs is most definitely carbohydrates--an excellent source of energy for cells to utilize. It really lacks protein and lipids, but is high in carbohydrates and of course, water, a necessary component of the body.