Human digestive juices will not be able to process cellulose which means that the answer is B. Cellulose can still be helpful to the system even though this cannot be broken down by the body. What it does is it will help the stools to have more bulk so that they can be passed by the body in an easier manner. Without the bulk, the movement of the intestines will not be as fast or will not be as effective.
Take note that a lot of the food that people eat do not contain the cellulose that we cannot consume anyway. There are some animals who are able to digest cellulose and make use of the benefits that they can get from it.