The correct answer is Unsustainable.
These diets are taken for a short period of time to reduce the weight of the body. They are used as a "quick fix" for the body. This is considered as the unsustainable diet because they can be consumed for a very shorter duration as it has only food to be eaten again and again, one cannot eat a single food for a longer duration of time. Though it can reduce weight or cure disease but it is unsustainable as it cannot be consumed much for a loner rime period.
example :cabbage soup